Percent Composition Calculator

Calculate the percent composition by mass of each element in a compound. Enter any chemical formula to see the complete breakdown.

Enter chemical formula with proper capitalization

Formula: % Element = (Mass of Element / Molar Mass of Compound) × 100 Where: Mass of Element = Atomic Mass × Subscript Molar Mass = Σ (Atomic Mass × Subscript) for all elements Steps: 1. Find atomic masses of all elements (periodic table) 2. Multiply each by its subscript 3. Sum to get molar mass 4. Divide element mass by molar mass × 100
Example 1: Carbon dioxide (CO₂) Atomic masses: C = 12.01, O = 16.00 Molar mass = 12.01 + (2 × 16.00) = 44.01 g/mol %C = (12.01 / 44.01) × 100 = 27.29% %O = (32.00 / 44.01) × 100 = 72.71% Example 2: Glucose (C₆H₁₂O₆) M = (6×12.01) + (12×1.008) + (6×16.00) M = 72.06 + 12.10 + 96.00 = 180.16 g/mol %C = 72.06/180.16 × 100 = 40.00% %H = 12.10/180.16 × 100 = 6.72% %O = 96.00/180.16 × 100 = 53.28% Example 3: From mass 5.0 g Na in 10.0 g compound %Na = 5/10 × 100 = 50%

What is percent composition in chemistry?

Percent composition tells you the mass percentage of each element in a compound. For example, water (H₂O) is 11.19% hydrogen and 88.81% oxygen by mass. It's calculated by dividing the total mass of each element by the compound's molar mass, then multiplying by 100.

How do I calculate percent composition from a chemical formula?

For each element: 1) Find the atomic mass from the periodic table, 2) Multiply by the subscript in the formula, 3) Add all atomic masses to get molar mass, 4) For each element: (atomic mass × subscript / molar mass) × 100. For C₆H₁₂O₆: %C = (72.07/180.16) × 100 = 40.00%.

How is percent composition related to empirical formulas?

Percent composition is used to determine empirical formulas: 1) Convert percentages to moles, 2) Divide by smallest mole value to get ratio, 3) If not whole numbers, multiply by smallest integer. This gives the simplest whole-number ratio of atoms in the compound.

Can percent composition be used to find molecular formulas?

Yes, with additional information. If you know the empirical formula and molar mass of the compound, divide the molar mass by empirical formula mass to get a factor n. Multiply subscripts in empirical formula by n to get molecular formula. Example: Empirical CH₂O (30 g/mol), actual 180 g/mol → n=6 → C₆H₁₂O₆.

What is the difference between percent composition and mass percent?

They're essentially the same thing - both express the mass fraction of each element as a percentage. Percent composition is the more general term referring to the entire breakdown, while mass percent might refer specifically to the mass percentage calculation. Both use the formula: (element mass / compound mass) × 100.

Why is percent composition important?

Percent composition is essential for: determining empirical formulas, checking purity of compounds, calculating theoretical yield in reactions, determining water of crystallization in hydrates, and understanding the elemental makeup of materials in chemistry, biochemistry, and environmental science.

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