The mass fraction of calcium hydroxide in a solution obtained by dissolving 0.03 g of calcium oxide in 25 cm ³ of water is …

Given:
m (CaO) = 0.03 g
V (H2O) = 25 cm3 = 25 ml

To find:
ω (Ca (OH) 2) -?

1) CaO + H2O => Ca (OH) 2;
2) n (CaO) = m / M = 0.03 / 56 = 0.00054 mol;
3) m (H2O) = ρ * V = 1 * 25 = 25 g;
4) n (H2O) = m / M = 25/18 = 1.38889 mol;
5) n (Ca (OH) 2) = n (CaO) = 0.00054 mol;
6) m (Ca (OH) 2) = n * M = 0.00054 * 74 = 0.03996 g;
7) m solution (Ca (OH) 2) = m (CaO) + m (H2O) = 0.03 + 25 = 25.03 g;
8) ω (Ca (OH) 2) = m * 100% / m solution = 0.03996 * 100% / 25.03 = 0.16%.

Answer: The mass fraction of Ca (OH) 2 is 0.16%.