Determine the mass fraction of the solute if: sodium hydroxide in an amount of 0.3 mol was dissolved in 80 ml water

Given:
n (NaOH) = 0.3 mol
V (H2O) = 80 ml

Find:
ω (NaOH) -?

1) M (NaOH) = Mr (NaOH) = Ar (Na) * N (Na) + Ar (O) * N (O) + Ar (H) * N (H) = 23 * 1 + 16 * 1 + 1 * 1 = 40 g / mol;
2) m (NaOH) = n * M = 0.3 * 40 = 12 g;
3) m (H2O) = ρ * V = 1 * 80 = 80 g;
4) m solution (NaOH) = m (NaOH) + m (H2O) = 12 + 80 = 92 g;
5) ω (NaOH) = m * 100% / m solution = 12 * 100% / 92 = 13%.

Answer: The mass fraction of NaOH is 13%.