solution to #20 on bonus PTCB practice quiz 5

by Darcetha
(Missouri)

Hello. As usual, I was over analyzing question 20 on the
Bonus free practice quiz 5(Opens in new window)

After putting it down and coming back to look at it, I finally figured out what I was doing wrong.

Question 20 wanted to know if 350ml of SWI is added to 75ml of an 85% stock solution, what is the percentage strength of the resulting solution?

In order to solve this problem, one would use the pharmacy dilution formula, which is C1V1=C2V2.

C1=concentration of solution 1
V1=volume of solution 1
C2=concentration of solution 2
V2=volume of solution 2

Using the above formula, you would put in the following numbers:

(350)(15%)=(75)(85%)

52.5=63.75

Divide both sides by 52.5 to get the answer 1.214.

So, the answer to this question is (B) or 15%.
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Reply (Keith)

You're on the right track, but forgot to put in your unknown.

The setup would go like this:


350(.00)+75(.85) = 425(X)



63.75 = 425X
----- = -----
  425     425


X = 0.15