Relative Formula Mass
Year 10 (IGCSE) ⚛️ Particles & Structure Calculate Mr for compounds; determine percentage composition by mass.
⚖️ Relative Formula Mass (Mr)
Mr = sum of the relative atomic masses of all atoms in the formula.
Key Ar values: H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, K=39, Ca=40, Fe=56, Cu=63.5
Mr(H₂O) = 2+16 = 18 | Mr(NaCl) = 58.5 | Mr(H₂SO₄) = 98 | Mr(CaCO₃) = 100
Mr(H₂O) = 2+16 = 18 | Mr(NaCl) = 58.5 | Mr(H₂SO₄) = 98 | Mr(CaCO₃) = 100
📊 Percentage Composition
% by Mass
$$\% = \frac{n \times A_r(\text{element})}{M_r(\text{compound})} \times 100$$% O in H₂O: (16/18)×100 = 88.9%
% S in H₂SO₄: (32/98)×100 = 32.7%
% S in H₂SO₄: (32/98)×100 = 32.7%
📈 Yield & Atom Economy
Key Formulas
$$\% \text{ yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$
$$\text{Atom economy} = \frac{M_r(\text{desired product})}{M_r(\text{all products})} \times 100$$ High atom economy = less waste = more sustainable (green chemistry).
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Percentage Composition by Mass