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Balanced equations and limiting reactant

A chemical equation is balanced when there are equally many atoms of each element on both sides. When two or more substances react, the limiting reactant is the one used up first and thus determines how much product can form — found by converting to moles and comparing with the coefficients in the balanced equation.

\mathrm{CH_4 + 2O_2 \to CO_2 + 2H_2O}

example of a balanced equation: C, H and O match on both sides

Example

32 g CH4 ( $M = 16.04$ ) gives $n = 1.995$ mol, giving the same number of moles of CO2 ( $M = 44.01$ ):

$1.995\cdot 44.01 \approx 87.8$ g CO2.

Always compare moles, not grams, when finding the limiting reactant — the equation's coefficients apply to moles.

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Part of General Chemistry: Moles and stoichiometry.