How many grams of a 15% phosphoric acid solution is required to completely neutralize 7.4 g of calcium hydroxide.

Given:
ω (H3PO4) = 15%
m (Ca (OH) 2) = 7.4 g

To find:
m solution (H3PO4) -?

Decision:
1) 2H3PO4 + 3Ca (OH) 2 => Ca3 (PO4) 2 + 6H2O;
2) M (Ca (OH) 2) = 74 g / mol;
3) n (Ca (OH) 2) = m (Ca (OH) 2) / M (Ca (OH) 2) = 7.4 / 74 = 0.1 mol;
4) n (H3PO4) = n (Ca (OH) 2) * 2/3 = 0.1 * 2/3 = 0.07 mol;
5) M (H3PO4) = 98 g / mol;
6) m (H3PO4) = n (H3PO4) * M (H3PO4) = 0.07 * 98 = 6.86 g;
7) m solution (H3PO4) = m (H3PO4) * 100% / ω (H3PO4) = 6.86 * 100% / 15% = 45.7 g.

Answer: The mass of the H3PO4 solution is 45.7 g.