Each chemical subscript scales an element’s contribution to one mole of the compound. Multiply the element’s atomic mass by the number indicated in the formula, then include that contribution in the compound’s molar mass. Using the subscript rather than the element’s atomic mass alone is essential because the formula may contain multiple atoms of the same element.
The approximately 100% total functions as an internal check on the calculation and the chemical formula used. If the values do not add to about 100%, at least one elemental contribution, molar-mass calculation, percentage, or formula entry should be reexamined. This check is useful before applying the composition to further chemistry calculations.
Experimental composition data can be compared with the percentages calculated for candidate formulas. Agreement between the measured and calculated elemental proportions helps identify an empirical formula and can contribute to determining a molecular formula. Thus, mass percent composition connects laboratory measurements with the symbolic formula used in subsequent molar and stoichiometric work.
Start by listing each constituent element and its subscript. Use each atomic mass and subscript to find that element’s mass contribution in one mole, then add all contributions to obtain the compound’s molar mass. Divide each contribution by the molar mass, multiply by 100, and check that percentages total approximately 100%.
Calculate the elemental percentages predicted by the proposed formula and compare them with the composition expected or obtained for the compound. The pattern of percentages provides a quantitative test of whether the formula represents the stated elemental makeup. A total near 100% also supplies a separate arithmetic consistency check.
Elemental percentages relate a compound’s overall mass to the amounts contributed by its constituent elements. That relationship lets chemists connect a compound’s formula and molar mass with the quantities of substances involved in a reaction. Consequently, composition data can help translate between elemental makeup and the amounts used or produced in stoichiometric work.