Percentage Strength Volume in Volume Dilution Pharmacy Math Question

by Ray
(McKinleyville, CA, USA)

I am a math idiot and I'm sure this is probably an easy question but here it is. What is the percent strength of a solution that is made by adding 200mL of sterile water to 600mL of a 25% solution?

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by: David

One quarter (25%) of 600 = 150
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starstarstarstarstar 		need more explanation
by: Anonymous

please explain going from 25%/600ml to 150/600ml, thanks
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starstarstarstarstar 		25% of volume is added to the solution; and percentage strength is diluted by 25%
by: David

First of all, thank you, Ray, for this question!

Next, we calculate the amount (in MG) of diluent in the solution:

25%/600ml = 150mg/600mL

Currently we have

150mg solute in 600mL solution.

Add the 200mL to the solution, which dilutes it...

150mg/800mL =

18.75% is the percentage strength of the newly diluted solution.

25% of 25% = 6.25%

25% - 25% = 18.75%

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