# NAPLEX Review Sample Math Question...

Discussion in 'Pharmacy Licensure and Exams' started by apharmx, Jun 21, 2008.

1. ### apharmx 2+ Year Member

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How many mL of water should be added to 100 mL of a 1:125 (w/v) solution to make a solution such that 25 mL diluted to 100 mL will yield a 1:4000 solution?

2. ### DrugDealer Senior Member 5+ Year Member

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Working backwards from the final solution to the intermediate solution (and using our old friend C1V1 = C2V2):

(25 mL)(1/x) = (100 mL)(1/4000)
x = 1000
so the concentration of the intermediate solution is 1/1000 (or 1:1000.....whatever)

And then going from the starting solution to this intermediate solution:

(100 mL)(1/125) = (x mL)(1/1000)
x = 800 mL of this intermediate solution
and since we started with 100 mL of the 1/125 solution......we need to add 700 mL to get to the 800

so my answer would be 700 mL

3. ### apharmx 2+ Year Member

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That's what I got as well....but I used a different method

1/125=x/25
x=.2g

1/4000=.2/x
x=800

800-100 (original)=700 ml

Thanks