Calculate the mass of sodium hydroxide if 11.5 g of sodium has reacted with water.

Let’s compose the reaction equation.
Let’s find the quantitative ratios of substances.

2 Na + 2H2O = 2NaOH + H2 ↑.

According to the reaction equation, 2 mol of sodium accounts for 2 mol of sodium hydroxide. Substances are in quantitative ratios 2: 2 = 1: 1.

The amount of substance will be the same.

n (Na) = n (NaOH) = 0.5 mol.

Find the mass of sodium hydroxide by the formula:

m = n × M,

M (NaOH) = 23 + 16 + 1 = 40 g / mol.

m = 0.5 mol × 40 g / mol = 2 g.

Answer: 20 g.