A medical office needs 525 mL of a 1:10 solution of hypochlorous acid. The stock solution is hypochlorous acid 25%. How much stock solution and how much water should be combined?

a. 310 mL stock and 215 mL water

b. 315 mL stock and 210 mL water

c. 210 mL stock and 525 mL water

d. 210 mL stock and 315 mL water

option d.

Explanation:

We can solve this exercise using the general expression for moles of two solutions which is:

n1 = n2

n is moles, in both solutions, even when they have different concentrations, the number of moles remain the same, and we know the concentration of each solution and their volume so:

n1 = C1*V1

n2 = C2*V2

Let 1 be the solution needed and 2, the stock solution so:

C1V1 = C2V2

The volume we get here would be the volume of the solute needed to reach the needed solution. - in other words, the volume of stock solution needed so:

1:10 it's the same as 0.1 and 25%, can be expressed as 25/100 = 0.25, therefore:

0.1 * 525 = 0.25 * V2

V2 = 0.1 * 525 / 0.25

V2 = 210 mL of stock solution needed

This means that to complete to 525 mL, we need to add 315 mL of water. And this is the final answer.