How much pure acid should be mixed with 4 gallons of a 50% acid solution in order to get an 80% acid solution?

Question

How much pure acid should be mixed with 4 gallons of 50% acid solution in order to get an 80% acid solution?

Answer 1

Pure acid can be considered to be 100% acid. Thus, you have 2 solutions: 100% acid and 50% acid. You want to mix these to obtain a solution that is 80% acid.

 

(x gal)((100%) + (4 gal)(50%) = (4 + x gal)(80%)

100x + 200 = 320 + 80x

20x = 120

x = 6 gallons