Composition Stoichiometry
Definition and meaning of Composition Stoichiometry in chemistry.
Composition stoichiometry is the use of mass relationships among the elements within a single compound to calculate quantities such as percent composition, empirical formulas, and molecular formulas.
In more detail
It rests on the law of definite proportions: a pure compound always contains its elements in the same fixed mass ratio, no matter the sample size. Starting from percent composition or measured masses, chemists convert grams of each element to moles using molar mass, then divide by the smallest mole value to obtain the simplest whole-number atom ratio, the empirical formula. If the compound's actual molar mass is known, the empirical formula can be scaled up to the true molecular formula. This differs from reaction stoichiometry, which relates amounts of separate reactants and products across a balanced chemical equation.
Key facts
| Field | General Chemistry |
|---|---|
| Governing law | Law of definite (constant) proportions |
| Determines | Percent composition, empirical formula, molecular formula |
| Contrasts with | Reaction stoichiometry (relates reactants and products) |
A compound is 40.0% C, 6.7% H, and 53.3% O by mass. In a 100 g sample: 40.0 g C ÷ 12.01 g/mol = 3.33 mol C; 6.7 g H ÷ 1.008 g/mol = 6.65 mol H; 53.3 g O ÷ 16.00 g/mol = 3.33 mol O. Dividing each by 3.33 gives a 1:2:1 ratio, so the empirical formula is CH2O; if the molar mass is found to be 180 g/mol, the molecular formula is C6H12O6 (glucose).
Frequently asked questions
How is composition stoichiometry different from reaction stoichiometry?
Composition stoichiometry works within one compound, using element mass ratios to find formulas, while reaction stoichiometry uses a balanced equation's mole ratios to relate the amounts of different substances consumed or produced in a reaction.
Can composition stoichiometry give the molecular formula directly?
Not by itself. It first yields the empirical formula from mass or percent data; the molecular formula requires also knowing the compound's molar mass so the empirical formula's mass can be scaled to match it.