Calculate the mass of sodium hydroxide required to obtain a 20% solution when dissolved in 250 ml of water.

Given:
ω (NaOH) = 20%
V solution (H2O) = 250 ml
ρ (H2O) = 1 g / ml

To find:
m (NaOH) -?

1) Calculate the mass of H2O:
m (H2O) = ρ (H2O) * V (H2O) = 1 * 250 = 250 g;
2) Let m (NaOH) = (x) r;
3) Make up an equation of two formulas for finding the mass of the solution and solve it;
m solution (NaOH) = m (NaOH) * 100% / ω (NaOH) = x * 100% / 20% = (5x) g;
m solution (NaOH) = m (NaOH) + m (H2O) = (x + 250) g;
x + 250 = 5x;
x = 62.5;
4) Determine the mass of NaOH:
m (NaOH) = x = 62.5 g.

Answer: The mass of sodium hydroxide is 62.5 g.