How much water must be added to a 25 mL sample of 1.5 M sodium sulfate in order to make it 0.20 M?

162.5 mL

Explanation:

M1*V1 = M2*V2

M - molarity

V - volume of solution

25*1.5 = 0.20 * V2

V2 = (25*1.5) / 0.20 = 187.5 mL of solution 2.

We had 25 mL of solution 1, solution 2 should have volume 187.5 mL,

so we have to add V2 - V1 = 187.5 - 25 = 162.5 mL of water, or ≈ 163 mL