2NaOH + H2S → 1Na2S + 2H2O
M (NaOH) = 40 г/моль
M (Na2S) = 78 г/моль
40 / 2 × 40 = X / 1 × 78
40 / 80 = X / 78
X = 40 × 78 / 80 = 39 г
