NAPLEX Review Sample Math Question...

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

  1. apharmx

    Joined:
    Jun 13, 2008
    Messages:
    29
    Likes Received:
    0
    Status:
    Pharmacy Student
    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. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. DrugDealer

    DrugDealer Senior Member

    Joined:
    Dec 19, 2004
    Messages:
    538
    Likes Received:
    1
    Status:
    Pharmacist
    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
     
  4. apharmx

    Joined:
    Jun 13, 2008
    Messages:
    29
    Likes Received:
    0
    Status:
    Pharmacy Student
    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
     

Share This Page