Chemical Change

Quantitative aspects, stoichiometry, limiting reagent

Quantitative Aspects of Chemical Change

Stoichiometry uses balanced equations and the mole concept to calculate quantities. Key: molar mass (M) in g/mol, n = m/M, concentration c = n/V, molar volume at STP = 22.4 dm cubed/mol. The limiting reagent determines maximum yield.

Example

Worked Example

In 2H2 + O2 -> 2H2O, if 4 g H2 reacts with excess O2: n(H2) = 4/2 = 2 mol. From the equation, 2 mol H2 produces 2 mol H2O. m(H2O) = 2 x 18 = 36 g of water.

Note

Exam Tip

Start with a balanced equation. Convert mass to moles, use mole ratios, convert back. Identify the limiting reagent when amounts of both reactants are given.

Key Vocabulary

StoichiometryThe calculation of quantities in chemical reactions using mole ratios

Limiting reagentThe reactant completely consumed first, limiting the amount of product

Molar massThe mass of one mole of a substance, in grams per mole

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Stoichiometry

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