1.4 Composition of Mixtures

Describing mixtures quantitatively

A mixture contains two or more substances physically combined, not chemically bonded. Composition is often expressed as mass percent = (mass of component/total mass) × 100, or molarity (M) = moles of solute/liters of solution. These formulas let chemists calculate exact amounts of each component in a solution or other mixture.

Dilutions and molarity calculations

When a solution is diluted, moles of solute stay constant while volume increases, so concentration drops. Use M₁V₁ = M₂V₂ to find new concentrations or volumes. This relationship is essential for preparing solutions of desired molarity from concentrated stock solutions.

Percent composition from formulas or data

Percent composition by mass can be calculated from a chemical formula (mass of element/molar mass of compound × 100) or from experimental mass data. This is used to verify empirical formulas and to determine purity of a mixture or compound.

Key terms

Molarity (M)

Moles of solute divided by liters of solution; a measure of solution concentration.

Mass percent

(Mass of component/total mass of mixture) × 100; expresses composition of a mixture.

Dilution equation

M₁V₁ = M₂V₂; used to find new concentration or volume when solvent is added.

Stock solution

A concentrated solution used to prepare more dilute solutions through dilution calculations.

Percent composition

The percentage by mass of each element in a compound, calculated from formula or experimental data.

Try an AP-style question

What distinguishes a mixture from a pure compound?

1. It always contains only one element
2. It has a fixed mass ratio
3. Its composition can vary
4. It can't be separated physically