Lesson 10 of 12 · 11 min

Stoichiometry and the limiting reagent

NCERT § "Stoichiometry and Stoichiometric Calculations"

This time the vessel holds 4 mol of hydrogen and 3 mol of oxygen. Spark it. How much water forms, and what is left over?

The lesson in notes

In short

Stoichiometry is the calculation of the masses and, for gases, volumes of reactants and products from a balanced equation.

The coefficients in a balanced equation give mole ratios (and, for gases at the same temperature and pressure, volume ratios), not mass ratios.

Balancing an equation means making the count of every element match on the left and right sides; this is conservation of mass written in symbols.

The limiting reagent is the reactant that is used up first; it alone decides how much product forms, and the other reactant is left in excess.

To find it, divide the available moles of each reactant by its coefficient; the smallest quotient marks the limiting reagent.

When a sample is impure, first find the mass of the pure reacting substance from its percentage purity, then do the mole calculation.

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