What mass of KOH must be dissolved in 150 g of water to obtain a solution with a mass fraction of 25%?

Given:
m (H2O) = 150 g
ω (KOH) = 25%

To find:
m (KOH) -?

Decision:
1) Calculate the mass fraction of H2O in the KOH solution:
ω (H2O) = 100% – ω (KOH) = 100% – 25% = 75%;
2) Calculate the mass of the KOH solution:
m solution (KOH) = m (H2O) * 100% / ω (H2O) = 150 * 100% / 75% = 200 g;
3) Calculate the mass of KOH in solution:
m (KOH) = m solution (KOH) – m (H2O) = 200 – 150 = 50 g.

Answer: The mass of KOH is 50 g.