A pharmacist has a 12% solution of boric acid and a 20% solution of boric acid. How much of each must he use to make 80 grams of a 15% boric acid solution?

Question

?  grams of 12% solution of boric acid and  ? grams of 20% solution of boric acid.

Answer 1

Assuming you meant 85ml instead of grams, we can proceed as follows.

 

C1V1 + C2V2 = C3V3, where C = concentration, and V = volume

 

we know that V1 + V2 = 85ml, so, V1 = 85ml - V2

changing the percent figures to decimals, we get

0.20V1 + 0.12(85ml - V2) = 0.15•85ml

0.2V1 + 10.2ml - 0.12V1 = 12.75ml

combining, we get 0.08V1 = 2.55ml

solving, we get V1 = 31.875ml

subtracting, V2 = 85ml - 31.875ml = 53.125ml

 

Proof: 0.2 31.875 + 0.12 53.125 = 12.75

6.375 + 6.375 = 12.75