Question

A chemist mixes an 8% hydrochloric acid solution with a 5% hydrochloric acid solution. How many millilitersof the 5% solution should the chemist use to make a 750-milliliter solution that is 7% hydrochloric acid?

Answer 1

Let x be volume of 5% acid and y be volume of the 8%


x + y = 750  and 
0.05x + 0.08y = 0.07*750 =  52.5

from first equation y = 750 - x so substituting in the second equation:-

0.05x + 0.08(750 - x) = 52.5
0.05 - 0.08x = 52.5 - 60
-0.03x = -7.5
x =  -7.5 / -0.3 =  250 mls  <------ answer