Step 1
% by mass:
Mass of solute ---- 100 of solution
Mass of solution = mass of solute + mass of solvent = 55.0 g + 137 g = 192 g
Solute = NH4Cl
Solvent = Water (H2O)
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Step 2
Procedure:
55.0 g NH4Cl --------- 192 g solution
X ---------- 100 g solution
X = 29 g NH4Cl = 29 % by mass approx.
Answer: 29 % by mass