How many ml of 50 percent acid should be added to pure acid to make 70ml of 65 percent acid?

91 mL

Explanation:

In chemistry, to solve this kind of exercises, dilutions, and concentrations is common to use the following equation

C1*V1=C2*V2

In which V1 refers to volume 1, in this case, is 70mL because is the quantity I have, the solution I can use to obtain another solution. C1 refers to the concentration of the solution I also have, in this case, 65%.

C2 and V2 refer to the volume and concentration of the solution I am looking for. So, if want to know V2, which is the volume I need to add, the only thing to do is find V2 in the equation:

V2=C1*V1/C2

V2 = 65%*70mL/50%

V2 = 91mL