How can I find the percent composition of (NH4)2S?

You calculate the mass of each atom in a formula unit, divide by the formula mass, and multiply by 100 %.

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For calculation purposes, we can write the formula of (NH₄)₂S as N₂H₈S.

Mass of N = 2 atoms N × ##(14.01"u")/(1"atom N")## = 28.02 u

Mass of H = 8 atoms H × ##(1.008"u")/(1"atom H")## = 8.064 u

Mass of S = 1 atom S × ##(32.06"u")/(1"atom S")## = 32.06 u

Mass of (NH₄)₂S = (28.01 + 8.064 + 32.06) u = 68.144 u

% S =##(28.02"u")/(68.144"u")## × 100 % = 41.12 %

% H =##(8.064"u")/(68.144"u")## × 100 % = 11.83 %

% N =##(32.06"u")/(68.144"u")## × 100 % = 47.05 %