Percent Composition Calculator

Calculate percent composition by mass of elements in chemical compounds and formulas

Enter a chemical formula to calculate percent composition

Optional: Enter sample mass to calculate actual element masses

What is Percent Composition?

Percent composition by mass describes the relative amount of each element in a chemical compound. It represents the percentage of the total mass that each element contributes. This is calculated by dividing the total mass of each element in the formula by the molar mass of the compound and multiplying by 100%.

Formula:

% Element = (Total Mass of Element / Molar Mass of Compound) × 100%

Why Calculate Percent Composition?

  • Determine empirical formulas from experimental data
  • Verify the purity of chemical samples
  • Analyze unknown compounds in analytical chemistry
  • Calculate nutrient content in fertilizers and supplements
  • Quality control in pharmaceutical manufacturing
  • Environmental analysis of pollutants and contaminants

Step-by-Step Example: Sulfuric Acid (Hâ‚‚SOâ‚„)

Step 1: Calculate Molar Mass

Find the atomic mass of each element and multiply by the number of atoms:

  • H: 2 × 1.008 = 2.016 g/mol
  • S: 1 × 32.06 = 32.06 g/mol
  • O: 4 × 15.999 = 63.996 g/mol
  • Total: 98.072 g/mol

Step 2: Calculate Percent Composition

Divide each element's mass by the total molar mass:

  • %H = (2.016 / 98.072) × 100% = 2.06%
  • %S = (32.06 / 98.072) × 100% = 32.69%
  • %O = (63.996 / 98.072) × 100% = 65.25%

Step 3: Verify

The sum of all percentages should equal 100% (or very close, accounting for rounding):

2.06% + 32.69% + 65.25% = 100.00% ✓

Common Applications

Fertilizer Analysis

Fertilizers like ammonium nitrate (NH₄NO₃) are analyzed to determine nitrogen content:

  • • Molar mass: 80.043 g/mol
  • • N content: 2 × 14.007 = 28.014 g/mol
  • • %N = 35.00% by mass

Iron Content in Ore

Determining iron percentage in hematite (Fe₂O₃):

  • • Molar mass: 159.687 g/mol
  • • Fe content: 2 × 55.845 = 111.69 g/mol
  • • %Fe = 69.94% by mass

Pharmaceutical Purity

Verifying aspirin (C₉H₈O₄) composition:

  • • Expected %C: 60.00%
  • • Expected %H: 4.48%
  • • Expected %O: 35.52%

Environmental Testing

Analyzing lead content in lead(II) chromate (PbCrOâ‚„):

  • • Molar mass: 323.19 g/mol
  • • Pb content: 207.2 g/mol
  • • %Pb = 64.12% by mass

Relationship to Empirical Formula

Percent composition is the reverse process of finding a molecular formula. If you have experimental percent composition data, you can determine the empirical formula:

Example: Unknown Compound

A compound contains: 40.00% C, 6.71% H, 53.29% O

  1. Assume 100g sample: 40.00g C, 6.71g H, 53.29g O
  2. Convert to moles: C = 3.33 mol, H = 6.66 mol, O = 3.33 mol
  3. Divide by smallest: C = 1, H = 2, O = 1
  4. Empirical formula: CHâ‚‚O

References

Percent composition calculations are based on standard atomic weights from:

Note: This calculator uses standard atomic weights. The percentages should sum to 100% (small variations may occur due to rounding). For research requiring high precision, consult peer-reviewed sources and use additional significant figures.

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Atomic Mass CalculatorEmpirical Formula CalculatorMole Ratio CalculatorMolarity Calculator
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