Percent Composition Calculator

Percent composition by mass tells you what fraction of a compound's total molar mass is contributed by each element. It is one of the most fundamental analyses in chemistry because it links a compound's formula to measurable physical quantities, specifically the masses of the individual elements. The formula for each element is straightforward: multiply the number of atoms of that element in one formula unit by the element's molar mass to get the element's contribution to the molar mass, then divide by the compound's total molar mass and multiply by 100. The percentages of all elements must sum to exactly 100 percent. This calculator accepts up to six elements. For each element you enter a symbol (for labelling), the number of atoms in the formula and the molar mass of that element in grams per mole. Common molar masses: H = 1.008, C = 12.011, N = 14.007, O = 15.999, Na = 22.990, Cl = 35.45, Ca = 40.078. The calculator returns the molar mass contribution of each element, the compound's total molar mass and the percentage by mass for each element. The worked example is pre-filled for water (H2O) showing hydrogen at 11.19% and oxygen at 88.81%. Percent composition data are used to verify molecular formulas from synthesis, to identify unknowns by elemental analysis, and as the starting point for deriving empirical formulas.

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Enter up to 6 elements (symbol, number of atoms, molar mass g/mol):

Molar masses: H = 1.008, C = 12.011, N = 14.007, O = 15.999, Na = 22.990, Cl = 35.45, Ca = 40.078, Fe = 55.845, S = 32.06, P = 30.974.