Percent Composition of Compounds - Chemistry

1. Fundamental Concepts

Percent composition of a compound is the percentage by mass of each element in the compound. It describes the relative amount of each element that makes up the compound, and the sum of all elements’ percent compositions equals 100% (accounting for minor rounding errors).

2. Key Concepts

Formula: Percent composition of element = (Mass of element in 1 mole of compound / Molar mass of compound) × 100%

Molar mass dependency: Requires calculating the molar mass of the compound (sum of atomic masses of all atoms in the formula) and the total mass of each element in one mole of the compound (atomic mass of element × number of atoms of the element in the formula).

Empirical formula link: Percent composition data can be used to determine the empirical formula (simplest whole-number ratio of atoms) of a compound.

3. Examples

Easy

Calculate the percent composition of sodium (Na) in NaCl (sodium chloride).

Atomic mass: Na = 23.0 g/mol, Cl = 35.5 g/mol
Molar mass of NaCl = 23.0 + 35.5 = 58.5 g/mol
% Na = (23.0 / 58.5) × 100% ≈ 39.3%

Medium

Calculate the percent composition of carbon (C) in glucose (C₆H₁₂O₆).

Atomic masses: C = 12.0 g/mol, H = 1.0 g/mol, O = 16.0 g/mol
Molar mass of C₆H₁₂O₆ = (6×12.0) + (12×1.0) + (6×16.0) = 72 + 12 + 96 = 180 g/mol
Total mass of C = 6×12.0 = 72 g/mol
% C = (72 / 180) × 100% = 40.0%

Hard

Calculate the percent composition of oxygen (O) in calcium phosphate (Ca₃(PO₄)₂).

Atomic masses: Ca = 40.1 g/mol, P = 31.0 g/mol, O = 16.0 g/mol
Molar mass of Ca₃(PO₄)₂ = (3×40.1) + (2×31.0) + (8×16.0) = 120.3 + 62 + 128 = 310.3 g/mol
Total mass of O = 8×16.0 = 128 g/mol
% O = (128 / 310.3) × 100% ≈ 41.25%

4. Problem-Solving Techniques

Step 1: Identify the compound’s chemical formula – Confirm the number of each atom (pay attention to parentheses, e.g., (PO₄)₂ means 2 P and 8 O atoms).

Step 2: Calculate molar mass – Sum the atomic masses of all atoms (use periodic table values; round to 1 decimal place for consistency).

Step 3: Compute total mass of the target element – Multiply the element’s atomic mass by its number of atoms in the formula.

Step 4: Apply the percent composition formula – Divide the element’s total mass by the compound’s molar mass, multiply by 100%, and round to a reasonable number of significant figures (matching the least precise atomic mass used).

Step 5: Verify – Check that all percent compositions add up to ~100% to catch calculation errors.