SCX Chemistry · Chapter 8: Percentage composition and formulas

Year 11 SCX · About 16 minutes with activities · Turn your sound on

Chapter 8 · Composition and formulas

Mystery compoundanalysed by mass

Mass ≠ atom countN atoms are heavier

Same ratio, same %

Puzzle solved!it’s glucose

Chapter 8 recap

Percentage composition = mass of the element in the formula ÷ M(compound) × 100. NH₃: 14.0 ÷ 17.0 × 100 = 82.35% N, 17.65% H.
Empirical formula (simplest whole-number ratio): assume 100 g → n = m ÷ M → divide by the smallest → round. Ratio of 1.5? Multiply all by 2 first. 5.9% H, 94.1% O → HO.
Molecular formula: M(molecular) ÷ M(empirical) = factor, then multiply every subscript. CH₂O (30.0) with M = 180 → 180 ÷ 30.0 = 6 → C₆H₁₂O₆.
Next: Chapter 9 · Gravimetry, hydrated salts and titrations

Same percentages, different compounds: how do you tell them apart?

Chapter 8 · About 16 minutes · sound on

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