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Pharmacy Calculations for Technicians - Concentrations and Dilutions

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This video explains a couple of methods to solve concentration and dilution problems in the pharmacy.
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Text Comments (133)
TheSergioization (14 hours ago)
This helped so much! Thank you.
Brad Wojcik (13 hours ago)
You are welcome! Glad I could help.
AmHie Zhing (3 months ago)
Hi dr. Brad I have a problem here.. Cefepime HCl is available asa 1-g vial for reconstitution. The question is how many mL of diluent would need to be added to reconstitute to concentration of 100mg/mL?
Brad Wojcik (3 months ago)
If you have more questions you can email me at brad.wojcik@gmail.com
AmHie Zhing (3 months ago)
Brad Wojcik I got my question from my workbook, it’s kind a tricky for me to do these calculations..
Brad Wojcik (3 months ago)
Hi Amhie. You really need a little more information to do this calculation. It turns out that Cefepime HCl forms a solution when reconstituted and the volume of the dry drug does not factor into the calculation. So it is just 1 g, which you can change off the bat to 1000 mg (1 mL/100 mg) = 10 mL. If you look on the prescribing info you will see that if you add 10 mL to a 1 g vial the approximate concentration is 100 mg/mL, but you will actually be withdrawing 10.5 mL from the vial. I really don't think it is a good question.
Brad Wojcik (6 months ago)
You can download my book, Pharmacy Calculations for Pharmacy Technicians: Solving Pharmacy Calculation Problems Without All the Silly Formulas, for free at https://payhip.com/b/5xVY. It goes into more detail than possible on the videos and is full of practice problems and answers.
Wasim Akram (8 months ago)
in what ratio 12%, 8% and 3% are mixed to get 5% alcohol........Sir give answer
Brad Wojcik (8 months ago)
Well, there are actually an infinite number of combinations of the three strengths if you wanted to use all three. I would just use 2 parts of 8% and 3 parts of 3%. One combination of all three is 1 part 8% one part 12% and 5 parts of 3%. To see how I got this, start by mixing equal parts of the 8 and 12% to make 10%.
Koo Laid (8 months ago)
I’m Brad Wojcik!! Not you!
Rebecca Howard (1 year ago)
This was exactly what I needed. Math always makes more sense when you watch someone else do it. Thank you so much!
Brad Wojcik (1 year ago)
Hi Rebecca. Glad I could help. If you have any questions you can email me at brad.wojcik@gmail.com.
liquidpersuasion (1 year ago)
C1V1 = C2V2. boom.
Brad Wojcik (1 year ago)
Not sure what your comment means. That was covered in the video.
Ggsk Sandika (2 years ago)
Thank you soo much Brad, you made my life. I have no words to explain how thankful I am.
Brad Wojcik (2 years ago)
Glad I could help. Let me know if you have any questions.
Melissa malv8689 (2 years ago)
hi there, I'm studying for my ptcb exam and im stuck on this dextrose problem and I really could use some help. I have my exam date for 8/15/16... the question is.. YOU ARE PLANNING TO PREPARE 600ML OF 20% DEXTROSE SOLUTION, BY MIXING YOUR 5% AND 50% DEXTROSE SOLUTION. HOW MUCH OF EACH SOLUTION WILL BE NEEDED??
Brad Wojcik (2 years ago)
Hi Melissa. This is an alligation problem. If you watch the alligation video at https://www.youtube.com/watch?v=CujuVNn7ROY it will explain how to do it. You will start the problem by drawing a box and putting 20 in the middle, 5 in the lower left corner, 50 in the upper left corner. If you want to email me at brad.wojcik@gmail.com I will show you how to do the problem. It will be pretty confusing without seeing the box and the calculations.
Ali Ali (2 years ago)
thanks  you sir u make easy and work for solutions  for  pharmacy calculation . thanks again your hard work.  u are very  smart way  to explain  pharmacy math  problems.
Brad Wojcik (2 years ago)
Thanks for the nice comment. Let me know if you have any questions.
macmakeuploverrr (2 years ago)
for the first question you went over.... is it 4ml stock and 96ml solvent? is that correct? or reverse? i understand the workings but the answers aren't written down by the actual questions itself
Brad Wojcik (2 years ago)
+macmakeuploverrr Hi Mac. It is 4 mL of stock and 96 mL of diluent. (We are not dissolving anything, so it is a diluent, not solvent. Thanks for watching.
Emily Hill (2 years ago)
Very helpful as I am currently in a pharmacy technician program, and the one thing that I seem to struggle with is when I am asked to find the active ingredient in a stock solution with a percentage or ratio strength. For example one of the questions I was having trouble with was: "Make 6 fl oz. (180mL) of a w/v stock solution so that one tablespoon (15mL) is diluted to 500mL produces a 1 : 2000 w/v solution (0.05%). How much active ingredient do you need for the stock solution? I tried many different methods and found out the answer was 3g but I don't understand.
Brad Wojcik (2 years ago)
+Emily Hill Well, try this one, so you can be sure you understand. Make 200 mL of a w/v stock solution so that 5 ml diluted to 1000 mL produces a 0.005% solution. How much active ingredient do you need for the stock solution? No hurry, I will be gone the rest of the day. Also, if you want, you can email me at brad.wojcik@gmail.com. It is easier to respond.
Emily Hill (2 years ago)
+Brad Wojcik Thank you, I think I understand it much better now. 
Brad Wojcik (2 years ago)
Yes, that is correct.
Emily Hill (2 years ago)
+Brad Wojcik Thank you for the help, I've been practicing questions like these and have found that with simpler stock solution calculations, I can easily use methods such as C1V1 = C2V2 but for ones like these finding the active ingredient or involving the aliquot I have to do two step proportions so it makes more sense. So if I understand this correctly, for a question like this, first we need to find out how much active ingredient is in the final dilution (500mL) with that final concentration of 1g/2000mL. Once we have that answer it is applied to how much is in one tablespoon (0.25g) that we add, so from there we have to find out how much of the active ingredient is in 180mL of what we need to dispense?
Brad Wojcik (2 years ago)
+Emily Hill Hi Emily. Yes, 3 g is correct. Before you can start doing the calculations on a problem like this you have to visualize the problem, or draw a picture. I pictured the 500 mL with the active ingredient and thought, "Ok, all the active ingredient came from that 15 mL of stock solution. How many g were in that 15 mL? 1:2000 is 1 g in 2000 mL. 500 mL(1 g/2000 mL) = 0.25 g. Ok, so we know that the stock solution is 0.25 g/15 mL. Now, how many g are in 180 mL of the stock solution? 180 mL (0.25 g/15 mL) = 3 g. You have to work backwards on problems like this. Let me know if you have any more questions. -Brad
Renna Sakka (2 years ago)
Wow what a great explanation, thank you!
Brad Wojcik (2 years ago)
+Renna Sakka Hi Renna, Glad you enjoyed it. Let me know if you have any questions. You can email them to me at brad.wojcik@gmail.com.-Brad
ayub yare (2 years ago)
hi doctor what is your webside.are the books free.
Brad Wojcik (2 years ago)
+ayub yare Here are the dropbox links where you can download the book and some exercises. Let me know if you have any questions. https://www.dropbox.com/s/eiuykndf06an11m/Milliequivalent%20Exercise.docx?dl=0 https://www.dropbox.com/s/jg5dywv9bqi2f4e/Pharm%20120%20Handouts%20Book%20third%20ed%20Jan%202013nh.docx?dl=0 https://www.dropbox.com/s/oqfe4nmfuwornp1/Pharm%20120%20Week%20One%20Exercises%20combined%20with%20answers.docx?dl=0 https://www.dropbox.com/s/oqfe4nmfuwornp1/Pharm%20120%20Week%20One%20Exercises%20combined%20with%20answers.docx?dl=0 https://www.dropbox.com/s/snw5jvu59e330w3/Pharm%20120%20Week%20Two%20Exercises%20combined%20with%20answersnh.docx?dl=0 https://www.dropbox.com/s/1bgzahdoyyafe7c/Powder%20Volume%20Exercise.docx?dl=0 https://www.dropbox.com/s/7hoqbtimvpo2kn8/Week%203%20Exercises%20combined%20with%20answersnh.docx?dl=0
Shilpa (2 years ago)
If 100 gms of dextrose is ordered using 50% dextrose solution, how many ml are needed? Thanks a bunch Dr. Brad!
Brad Wojcik (2 years ago)
+Shilpa Rivera Hi Shilpa. Read over the material I sent. The most important things to know is that the units (mg, g, mL, gtt, etc.) are the most important things. The numbers just go along for the ride. About half of all the problems are done the same way (conversions, dosage calculations, drip rates, and others). Once you learn the basic method, everything will be easy. It is all in the book. If you have questions, just email me at brad.wojcik@gmail.com. It is easier to respond in the email. You will do great.
Shilpa (2 years ago)
+Brad Wojcik Thank you so much!! Any other Pharmacy tech math link, please do post them here. Dr. Brad, I really appreciate your sincerity in helping us achieve our goal. Math has been a problem for me so all these times I put pharmacy tech state board exam behind. Now that I have found you, I am so hopeful that I will pass the exam. God bless!! Shilpa
Brad Wojcik (2 years ago)
+Shilpa Rivera Here are some links to additional problems. Also, there is a link to a book I put together. https://www.dropbox.com/s/eiuykndf06an11m/Milliequivalent%20Exercise.docx?dl=0 https://www.dropbox.com/s/jg5dywv9bqi2f4e/Pharm%20120%20Handouts%20Book%20third%20ed%20Jan%202013nh.docx?dl=0 https://www.dropbox.com/s/oqfe4nmfuwornp1/Pharm%20120%20Week%20One%20Exercises%20combined%20with%20answers.docx?dl=0 https://www.dropbox.com/s/snw5jvu59e330w3/Pharm%20120%20Week%20Two%20Exercises%20combined%20with%20answersnh.docx?dl=0 https://www.dropbox.com/s/1bgzahdoyyafe7c/Powder%20Volume%20Exercise.docx?dl=0 https://www.dropbox.com/s/7hoqbtimvpo2kn8/Week%203%20Exercises%20combined%20with%20answersnh.docx?dl=0 Let me know if you have any questions. -Dr. Brad
Shilpa (2 years ago)
+Brad Wojcik Thank you so much!! Do you have more problems like this on different link because I have few more problems that I dont seem to figure out.
Brad Wojcik (2 years ago)
+Shilpa Rivera Hi Shilpa. You are starting with 100 g (use g, not gms) of dextrose and have to end up with ml (you can write it ml or mL). The ratio is 50%, which is really 50% g/mL (watch the percentage video). 50% g/mL is 50 g/100 mL. You need mL in the answer, so you have to flip the ratio and use 100 mL/50 g. So--- 100 g (100 mL/50 g) = 200 mL. Hope this helps.
FunkoMomma (2 years ago)
Thanks so much for this video! I'm trying to get a head start on a future lecture, and this is exactly what I needed!
Brad Wojcik (2 years ago)
+Jennifer Fagan Hi Jennifer. Glad I could help. We have all done these types of problems in everyday life with no problem. If you have 12 lamps in your house and you need 2 bulbs for each lamp you need 24 bulbs. You go to the bulb store and look on the shelf and see that they come in boxes of 6 bulbs, so you get 4 boxes. This is the same as: "Make 12 mL of a solution of 2 mg/mL. Your stock solution is 6 mg/ mL, how many mL of stock solution do you need?"
Brandee Bell (2 years ago)
Thank you for this! Online classes can be really confusing and this really helped me understand what my teachers power point was explaining. I appreciate it!
Brad Wojcik (2 years ago)
+b belle Hi b belle. Glad I could help. Non of this material is that difficult if you just understand what you are actually doing. Let me know if you have any questions. It is easier if you email me at brad.wojcik@gmail.com
Cecille Boznango (2 years ago)
Thanks so much, Brad. Please add some alligation problems esp. those that ask how much of drug B should be added to A given the amount of B to yield C%.
Brad Wojcik (2 years ago)
+Cecille Boznango You are welcome. If you have a specific problem you would like help with, please email me at brad.wojcik@gmail.com.
Helen Barela (2 years ago)
Hi Brad, I wanted to say THANK YOU for your great videos!!!!
Ana Angel (3 years ago)
thank you. l really appreciate that you are helping me.
Brad Wojcik (3 years ago)
+Ana Angel I am always glad to help.
Ana Angel (3 years ago)
hi mr . Brad can u help me to solve this problem; an aminophylline drip is running at 1mg/kg per hour in a 12-kg child. how much aminophylline is the child receiving per day?
Brad Wojcik (3 years ago)
+Ana Angel Hi Ana. When you have a ratio like 1mg/kg/hr, you can rewrite it as 1 mg/kg*h, which makes it easier to work with. You have to end up with mg and they are telling you that the child weighs 12 kg and the drip is running 1 day. Let's change 1 day into hours. So the givens are 24 h and 12 kg, the ratio is 1 mg/kg*h and the units of the answer are mg. (24 h)(12 kg)(1mg/kg*h)= 288 mg. h and kg both cancel out and you are left with mg.
Arleene Perez (3 years ago)
Hi brad watching your video I'm still confused. here is a question that I have been really stuck with and if you could explain it to me. a norepinephrine drip contains 4mg in 250 ml D5W. what is the concentration in mcg/ml?
Brad Wojcik (3 years ago)
+Arleene Perez Hi Arleen. You are starting with mg/ml and have to end up with mcg/ml, so just change the 4 mg to mcg. (4 mg/250 ml)(1000 mcg/mg) = 16 mcg/ ml. See how the units of mg cancel out?. Hope this helps.
Sara Rodriguez (3 years ago)
Hi Mr. Wojcik. I was a bit confused on what the two parts in the solution are called and where in the pharmacy would this type of question would need to come into play? I am a bit confused on the last problem using the first method (not the formula). This may seem silly but why did you put the 100 into the problem under the1 %?  Thank you!
Sara Rodriguez (3 years ago)
+Brad Wojcik  Thank you so much!
Brad Wojcik (3 years ago)
+Sara Rodriguez Hi Sara. The two parts of a solution are the solute and the solvent. The solute is dissolved in the solvent. A diluent is added to make the solution weaker. I put 100% under the 1% to remove the %. There is a video on percentages. Basically, to add the % sign, multiply by 100%. 0.25 (100%)= 25%. To remove the % sign, divide by 100%. 25%/100%=0.25. You would use these calculations mostly in a compounding pharmacy.
yathatisgood (3 years ago)
Thank you so much. That really helped me. Kind regards Steve
yathatisgood (3 years ago)
If I have 14% sodium hypochlorite m/m How can I dilute it to make 2% solution?
Brad Wojcik (3 years ago)
+yathatisgood You don't say how many grams of 14% sodium hypochlorite you are starting with, so lets just say you are starting with 100g. This means you would have 14g NaOCl /100 g of solution. Now calculate how many grams of solution you would have if you had 14 g NaOCl in a 2% solution. You can set it up as 14g NaOCl/x = 2% Changing the 2% to a decimal you have 14 g/NaOCl/x = 0.02. Solving for x you get 700g. You started with 100g, so you would have to add 600 g of water.
eleno sigarroa silva (3 years ago)
Where did we get the 200?
Brad Wojcik (3 years ago)
+the batman Hi Batman. I assume you are referring to the 200 mg in the problem where we are making 100 ml of a 2 mg/ml solution. 100 ml ( 2mg/1ml)= 200 mg of active ingredient needed in the final solution, You will get those 200mg from your stock solution which has 50mg/ml. 200mg (1 ml/50mg)= 4 ml of stock solution. Hope this helps.
Georgegina McDonald (3 years ago)
Hi Brad Would you be able to do a demonstration video on aliquots?  
Georgegina McDonald (3 years ago)
Thank you Brad, that helps a lot :) Georgie
Brad Wojcik (3 years ago)
Hi Georgegina. It doesn't look like I am going to be able to make one today, but here is an excellent slide share presentation on the subject. http://www.slideshare.net/layon89/chap3-calculation. 
Brad Wojcik (3 years ago)
I will try to make one tomorrow. If you have a problem you would like help on, just email it to me at brad.wojcik@gmail.com
yasser martians (3 years ago)
please help me... If a pharmacist added 12 g of azelaic acid to 50 g of an ointment containing 15% of azelaic acid, what would be the final cocentration of azelaic acid in the ointment?
yasser martians (3 years ago)
+Brad Wojcik thank you .....this helped me too much
Brad Wojcik (3 years ago)
Hi Yasser,  First calculate how many grams of azelaic are in the 50 g of 15%. (0.15)(50g)=7.5 g. Now you are adding 12 g, so you will have a total of 19.5g of azelaic acid. The total amount of oint you have now is 62g (12g +50g). So you have 19.5g active ing/62 g oint. To change this to a %, multiply by 100%. (19.5g AI/62g oint)(100%)= 31.45%. Hope this helps.
Lauren Cannon (3 years ago)
How do you solve? 3g of Zovirax ointment contains 150mg of acyclovir. What is the concentration of this product?
Brad Wojcik (3 years ago)
Hi Lauren, First you have to know what the units of the answer will be. This is a %w/w problem because you are weighing out the active ingredient and you are weighing the final product. For % w/w the units will be % g active ing/g final product. You are starting off with 150 mg/ act ing/3 g final product. You have to change the mg to g and add the %. To change the mg to g, you multiply by 1 g/ 1000 mg. To add the % sign, you multiply by 100%. So: 150mg AI/3 g final prod (1 g AI/1000 mg AI)(100%) = 5%. See how the mg cancel out? Hope this helps. PS. Cute baby!
Melinda Jefferson (3 years ago)
Hi Sir,Can I ask you again to solve these problems:1)How many cm3 of 85% H3PO4 solution(d=1,689g/cm3),needs to be taken and how many cm3 of H2O should be added in order to get 2dm3 of solution that has a concentration of 0.2 mol/dm3?  2)How many cm3 Of H20 should be added in 300cm3 of 48%H2SO4(d=1,38g/cm3) in order to get a solution of 1,61mol/dm3 and d=1,10g/cm3?
Brad Wojcik (3 years ago)
Hi melinda, I am happy to help.
Melinda Jefferson (3 years ago)
Dear Sir, this is just great! I cannot thank you enough for helping me!  Sorry for bothering you! When I look at how much you have written It really makes me uncomfortable.. So I will definitely disappear for a while! 
Brad Wojcik (3 years ago)
#2)First calculate how many g of H2SO4 are in the 300 cm3. 300cc(1.38g/cc)(0.48)= 198.72g. The molecular weight of H2SO4 is 98.08 g/mol, so calculate how many g of H2SO4 will be in 1 dm3.   (98.08 g/mol)(1.61 mol/dm3)= 157.9 g/dm3. Now calculate how many cc of solution you can make with the 198.72 g that you started with in the 300cc.  198.72 g(1000cc/157.9g) = 1258.5 cc. You started with 300cc, so you have to add 958.5 cc.  When I calculated the final density, I came up with 1.09 g/cc. (414.0 g from the 300cc and 958.5 g from the H20 = 1372.5g/1258.5 cc which is 1.09 g/cc)
Brad Wojcik (3 years ago)
This is how I did it. First, you have to look up the molecular weight of H3PO4, which is 98g/mol. You know you need 0.2 mol/dm3 and you are making 2 dm3. This means that you will need 39.2 g of H3PO4 in the 2 dm3. 2 dm3(0.2mol/dm3)(98g/mol)=39.2 g. Now you look at where you will get your 39.2 g of H3PO4. You have 85% w/w H3PO4 with a density of 1.689g/cc. This means that there are 1.436 g/cc of H3PO4-- (0.85)(1.689 g/cm3)= 1.436 g/cm3. Now you can calculate how many cc of the 85% you will need to get 39.2 g.  39.2 g (1cc/1.436g) = 27.3 cc. The balance of the 2dm3 is H20, which is 1972.7 ml. 
Chantal Williams (3 years ago)
How many milliliters of a 10% potassium solution are needed to make 180 ml of a 10 mg/ml concentration solution?  The book has the answer and how to solve the problem but I don't know where the problem starts from. maybe it is how it is worded that I don't understand.  10 mg/ml =1000 mg/100 ml=1 g/100 ml= 1% ---->Where did the 1000mg/100ml come from? 1 g/100ml= x/180 ml x=(1 g X 180 ml)/100ml =1.8g needed 1.8 g/x =10 g/100 ml x=(1.8 g X 100 ml)/ 10 g = 18 ml Is this a % w/v problem or % w/w
Brad Wojcik (3 years ago)
That is the long way of doing it. First calculate how many grams of potassium you in in the 180 ml. 180 mL(10mg/mL)(1 g/1000 mL) = 1.8 g. Now calculate how many mL of the 10% solution you need to get 1.8 g. 10% is 10g/100mL (w/v)  1.8g(100mL/10g) = 18 mL. Just like you were baking cookies. You would calculate how many chocolate chips you need, then go to the store. 
Melinda Jefferson (3 years ago)
Hi Sir,could you please help me with these two problems: 1) How many dm3 of 16% solution of CuSO4 (d=1.18g/cm3) we can get from 590,5 g of CuSO4 * 5H2O?     2) How many grams of H2O we need to add in 0.25dm3 of 35,2% solution of Ammonia (d=0.88g/cm3) in order to get 3% solution?
Brad Wojcik (3 years ago)
Glad I could help.
Melinda Jefferson (3 years ago)
Thank you so much! You are amazing! It really means a lot to me!
Brad Wojcik (3 years ago)
2) The Ammonia solution is w/w, which they should tell you. First calculate how many grams in the 0.25dm3, 250 cc ( 0.88g/cc) = 220 g. Now calculate how many grams of Ammonia. 220g ( 35.2g/100g) = 77.44 g Ammonia. Now set up an equation 77.44 g = 0.03 (x) , where x is the total weight of the 3% solution. Solving for x you get 2580g. You have 77.44 g of Ammonia, so you have to add the difference, which is 2502.56 g of H20. Sorry that is is kind of hard to explain everything in detail in the space given. 
Brad Wojcik (3 years ago)
Hi Melinda. To begin with, they should tell you if the solutions are w/w or w/v. CuSO4 is usually w/v, so we will do it that way. First off you have to calculate how many grams of CuSO4 you can get from the 590.5 g of the pentahydrate form. You have to look up the atomic weights of both forms, then you can use those values to form a ratio of CuSO4/CuSO4-5H20.  590.5 g Pentahydrate (159.61 g CuSO4/249.69 g CuSO4-5H20) = 377.5 g CuSO4. Now you can calculate how many cc of 16% you can get from the 377.5 g   377.5 g ( 100 cc/16g) = 2359.4 cc, which is 2.359 dm3. Since they gave the density, they may be using 16% w/w, not w/v. You would use the ratio 100g/16g and you would end up with 2359.3 g., Now you change that to a volume. There is 1 cc/1.18 g.  2359.3 g(1cc/1.18g) = 1999.4 cc or roughly 2 dm3. Again, I believe they should have given you a little more information on this one. 
hi dr wojcik how will i do this problem. hydro cortisone 3% ointment 80g is added to enough ointment base to make a total of 1 pound. what is the new percent strength of hydro cortisone ointment?
yes it do thanks
Brad Wojcik (3 years ago)
The easiest way to do this is W1C1=W2C2. Weight1 (concentration)1= weight2 (concentration)2 80g (3%) = 454g (x)  x = 0.52%  Or you can calculate the number of grams of HC in the 80%, then you will know how many grams are in the 454 g. Then change back to a %.  80g(3%)= 2.4 g  Now you know you have 2.4 g in 454 g.   (2.4g/454g) 100% = 0.52% Hope this helps. 
lisal29 (4 years ago)
I am still stuck on tar and ointment calculation. how did you come up with the answer.
Brad Wojcik (4 years ago)
What is the question?
Christine Valentini (4 years ago)
Hi Dr Wojcik, can you please help me with this problem: Have 300ml of a 50% solution; 200ml is added to decrease concentration. How many active ingredient would be in 4oz of the diluted solution. Thank you so much. Christine
Brad Wojcik (4 years ago)
Let's assume it is w/v. Step 1) Calculate how many grams of active ingredient are in the 300 mL of 50%. Then you will have the same number of grams in 500 mL after you add the 200 mL. Then you will set up your problem 120 mL ( number of g in 500 mL/500 mL) = g You should get 36 g. Give it a try.
Kaitlin Mathewson (4 years ago)
Hello I have also found your videos extremely helpful I am also attending pharm tech school and working my way through pharmaceutical calculations and have a question. Folic acid is commonly available as a 5mg/ml injection. You are to make a preparation that contains 50mcg/ml in a total volume of ml. How would I set this problem up using v1c1=v2c2
Brad Wojcik (4 years ago)
What is the total volume of the 50 mcg/mL?
Christine Valentini (4 years ago)
Hi Dr. Wojcik, I am taking a Pharmacy Tech course online. Your video's have been so helpful for me. I really appreciate how you simplify the problems by using everyday terms. Please help with this prob. The order calls for 120ml of a 10% solution. We have a 25% of the same solution. How many mls of  the 25% solution will I use? Thank you, Chritine
Brad Wojcik (4 years ago)
Hi Christine. Thanks for the nice comment. There are a couple of different ways you can do this. Method 1) Lets assume it is a w/v problem. You are making 120 mL of a 10% solution. First determine how many grams you need in the 120 mL   120mL (10 g/100 mL) 12 g. Now you go shopping for your 12 g. You will get them from the 25% solution. 12 g ( 100 mL/25g) = 48 mL. If you don't know how to get  10 g/100 mL from a 10% solution, review the percent video. The other way is to use the formula V1C1=V2C2. That is volume 1 times concentration 1 = volume 2 times concentration 2. V1=120 mL C1 = 10% V2 = X    C2= 25%  120 mL (10%) = X (25%) solving for X we get 48 mL. I hope this helps. Dr. Brad
Brad Wojcik (4 years ago)
Hi Dent, I am happy I could help you. Be sure to download my book from the main channel page. Brad
Brad Wojcik (4 years ago)
lolo- could you please email your problems to me at brad.wojcik@gmail.com? I see you have a couple of problems, but I am having a problem trying to reply to them. 
mrsalscott (4 years ago)
Hi Dr. Brad. I might just be over thinking this problem but I am still needing some help. Your videos have help quite a bit so I must thank you for that. So the question is "How many mL must be added to 150mL of a 25%(v/w) stock solution of sodium chloride to prepare a 0.9%(v/w) sodium chloride solution? Any help would be great
Brad Wojcik (4 years ago)
Great job on the quiz.
mrsalscott (4 years ago)
Well sir, with your help, I received an A on my test and quiz. Thank you for all your help and patience. One day I will return the favor for another person in need. Just like you helped me in my time of need
Brad Wojcik (4 years ago)
It helps to stop an think about the problem first. If you are given some coal tar and told that you will be making a 1% ointment, the first thing you should think is that you will be able to make a lot of ointment. That will get you started on the right path.
mrsalscott (4 years ago)
Nope I was doing it completely wrong. Wow, so much to learn and yet I'm doing little mistakes but in medicine could mean a lot. Thanks for the guidance it sure does mean a lot!!!! 
Brad Wojcik (4 years ago)
The 15 g of coal tar will be equal to 1% of the final product. You will be able to make 1500 g of oint with the 15 g of coal tar. You would mix the 15 g of coal tar with 1,485 g of oint base. 
sarainreno30 (4 years ago)
So remembering rules instead of formulas works better in real pharmacy situations. Okay! I will have to watch this one again.
Brad Wojcik (4 years ago)
Yes, the way I look at it is that you can either try to remember a list of twenty plus formulas and hope that your particular problem will fit one of the formulas, or you can understand what you are actually doing so you can set the problem up and solve it on your own.
Kuldeepak Sharma (4 years ago)
Excellent job. Thanks for sharing valuable information.
Brad Wojcik (4 years ago)
10% w/v solution if you withdrew 10 mL and q.s to 100 mL, 20% w/v, 25% w/v, 35% w/v.
Brad Wojcik (4 years ago)
Percent strength for w/v is % g/ mL. You have g/mL now, so to add the %, just multiply by 100%. 1 g/100 mL (100%) = 1% g/mL or 1% w/v. Doing the other problems the same way you will get 2% w/v, 2.5% w/v, 3.5% w/v. Remember, start by calculating how much solute (the Na sal) you will end up with in the final solution, then change back to percent. Hope this helps and thanks for watching my videos. -Dr. Brad p.s if the stock solution was the 100 g/100 mL you calculate the same way and you would have
fatima almosa (3 months ago)
Brad Wojcik Thank you very much from KSA
Brad Wojcik (4 years ago)
Hi Ksenya, I am a little confused. Is it 10% w/v or 100 g in 100 mL? If it is 10% w/v that is 10%g/mL or 10 g in 100 mL of solution. It is best to go back to the basics and determine how many g of Na salicylate you have in your final solutions, then convert back to percentages. If you withdraw 10 mL of the 10 g/100 mL, you would have 1 g Na Sal in the 10 mL that you withdrew. 10 mL (10 g/100 mL) = 1 g. So you end up with 1 g in 100 mL after you add water up to 100 mL. Now convert back to %.
ksenya alotti (4 years ago)
hey Dr.brad i'm a pharmacy student and i always face problems in this would you please explain us how to calculate the concentration if you prepare for examplea .10gm% w/v (Na-salicylate in D. water) stock solution ( 100 gm Na salicylate and water up to 100 ml) and then withdraw 10, 20, 25, 35 ml from the stock solution transfer it to a test tube then complete the volume in each to up to 100 ml what would be these new concentrations?
Brad Wojcik (5 years ago)
Hi Stacy, Thanks for the nice comment. When I teach this course, I tell the students they don't have to memorize any formulas, but if they do want to learn one, learn V1C1=V2C2. You can do the problems without using the formula, but it really comes in handy. -Dr. Brad
Stacey Lewis (5 years ago)
You are a life saver! I was soooo lost and confused but V1C1=V2C2 makes things 100% easier. Thank you
Brad Wojcik (5 years ago)
Hi Garrett, Thanks for the nice comment. Yes, once you understand that most of these problems are nothing more than multiplying your given by one or more forms of 1 to change its appearance, the course is pretty easy. Let me know if you have any questions on any specific problems. -Dr. Brad
Garrett Sanders (5 years ago)
Thank you for taking a step back with this and showing us what the basis for the math is. To many programs just tell you the formulas and send you on your way!!
Brad Wojcik (5 years ago)
I posted some exercises and my handout book on the main page. I think it will help you if you download them and do the exercises.
Tanya Matyuk (5 years ago)
Dr Brad, thanks again for the tips!
Brad Wojcik (5 years ago)
Hi Matyuktanya, Yes, you will be doing many different types of conversions. All the conversions are preformed in the same manner, you just have to have the correct conversion factor. You will have to learn that 1 tsp = 5 mL, etc. Dr. Brad
Tanya Matyuk (5 years ago)
Dr Brad, Thank you that is very helpful, I should be able to learn these and do lots of practicing to get it down. Okay I'll see those links.. Also, a professor told me that in the pharmacy they use conversions for tbsp, oz, gallons and so on.. is that true?
Brad Wojcik (5 years ago)
I have posted links to some written material under channel posts. You should download the handout book.
Brad Wojcik (5 years ago)
Also, be sure to watch the video on unit conversions. If you learn to use conversion factors rather than moving the decimal point, you will be much less prone to error.
Brad Wojcik (5 years ago)
Hi Matyuktanya, There are a lot of prefixes in the metric system, but in pharmacy the ones that we use most are milli- (one thousandth), micro-(one millionth) and kilo- (one thousand). If you learn that 1 g =1000 mg, 1 mg=1000 mcg, 1 L = 1000 mL, 1 kg = 1000 g you will be able to do 99% of all the problems. You can remember that kilo- means 1000 by thinking about a kilometer (kilo-meter). It is 1000 meters, Look at a ruler marked in millimeters and you can remember that milli- is one thousandth.
Tanya Matyuk (5 years ago)
Dr. Brad, thank you for the videos for the first time I can somewhat understand the problems. I was wondering if there is a trick to remembering the conversions from mg to g and so on, I realized I need those memorized and I'm having a hard time. Thank you!
Brad Wojcik (5 years ago)
Hi Chris, Glad they help you. Let me know if you need any help on any problems.
Chris Harris (5 years ago)
I'm going through these again as we hit the math portions of Pharmacy 260, thank you so much for these videos Dr. Brad!
Brad Wojcik (5 years ago)
Thanks Igan!
igan esperancilla (5 years ago)
You are awesome dr.brad;) igan;)

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