Reacting masses and the limiting reactant

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The reactant that **runs out first** — it controls (limits) the amount of product that can form.

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The reactant **left over** once the limiting reactant has been used up.

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The **maximum** amount (or mass) of product, calculated from the **limiting** reactant.

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Convert each reactant mass to **moles**, divide each by its **coefficient**, and the **smallest** result is limiting.

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It compares the reactants fairly against the **mole ratio** in the balanced equation, so you can see which runs out first.

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Always the **limiting** reactant — never the one in excess.

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$n = \dfrac{m}{M}$ — convert every mass to moles before using the mole ratio.

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Balanced equation → mass to **moles** (n = m/M) → scale by the **mole ratio** → moles back to **mass** (m = nM).

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From the **coefficients** of the balanced equation (e.g. N_{2} + 3H_{2} → 2NH_{3} is 1 : 3 : 2).

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Working out the product from the reactant in **excess**, or forgetting the **mole ratio** when coefficients are not 1 : 1.

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**A** (0.3 mol runs out first); B is in excess by 0.2 mol.