The mass of gas evolved during the treatment with an excess of hydrochloric acid is equal to 3 mol of sodium sulfide.

1. Let’s compose the reaction equation, find the quantitative relations of substances.
Na2S + 2HCl = H2S ↑ + 2NaCl.
1 mole of sodium sulfide accounts for 1 mole of hydrogen sulfide. Substances are in quantitative ratios of 1: 1. The amount of the substance will be the same.
n (Na2S) = n (H2S) = 3 mol.
2. Let’s find the mass Н2S.
m = n M.
M (H2S) = 34 g / mol.
m = 3 mol × 34 g / mol = 102 g.
Answer: 102 g.