Question about Concentrations and Dilutions

by Daniel E.
(Redding CA)

My pharmacy technician class just recently started into the module of concentrations and dilutions. From the alligations and more of the easy stuff this type of math is starting to confuse the crap out of me. I never know where to start for all the different worded problems. I have an example question from the book I'm working from and would like to know where to start if possible.


Prepare 3 mL of a 10mg/mL phenobarbital solution from a 65mg/mL stock solution that comes in a 1 mL vial. How much concentrate and how much diluent are needed to obtain the desired final volume?

Thank you for your time

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Question about Concentrations and Dilutions

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Aug 01, 2011
Another solution (Alligation)
by: Anonymous

u can use the alligation method .. the stock conc. is 6.5 and for water is 0 while the required conc. is 1

Jul 15, 2011
by: Anonymous

3ml of concentration of 10 is equal to 0.46ml of concentration of 65. 3ml minus 0.46 will be the amount of diluent which is 2.54ml

Apr 24, 2011
by: Anonymous

How do you do get .46 and 2.54?

Apr 24, 2011
by: Anonymous

0.46 ml of the concentrate and 2.54 ml of diluent.
0.46 ml contains 30mg of the ingredient.
So, when diluted to 3ml we get 30mg/3ml or 10mg/ml.

65-10 = 55 parts
10-0 = 10 parts
55+10 = 65 parts (in total)
3ml/65 = 0.046ml = 1 part

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