% purity = (mass of pure substance ÷ mass of sample) × 100 · NaOH sample:
97% pure
, 3% Na₂CO₃ impurity
m(pure NaOH) = m(sample) × (97 ÷ 100) ·
Always apply purity before calculating moles
Purity
97%
NaOH (pure, 97%)
Na₂CO₃ (impurity, 3%)