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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

1 Atomic Structure

1.1 Moles and Molar Mass 1.2 Mass Spectra of Elements 1.3 Elemental Composition of Pure Substances 1.4 Composition of Mixtures 1.5 Atomic Structure and Electron Configuration 1.6 Photoelectron Spectroscopy 1.7 Periodic Trends 1.8 Valence Electrons and Ionic Compounds

Composition of Mixtures

Learning Objective 1.4.A Explain the quantitative relationship between the elemental composition by mass and the composition of substances in a mixture.

Quick Notes

  • A pure substance contains only one type of atom, molecule, or formula unit.
  • A mixture contains two or more substances in varying proportions.
  • The composition of a mixture is not fixed — it can be separated by physical methods.
  • Elemental analysis uses the percent composition by mass to:
    • Determine the ratio of atoms in a compound
    • Assess the purity of a sample by comparing expected and actual composition

Full Notes

Substances in chemistry can be classified as pure substances or mixtures, depending on their composition and uniformity.

Pure Substances

A pure substance contains only one kind of particle:

Pure substances have a fixed composition and consistent properties throughout the sample.

AP Chemistry diagram of a pure substance showing only water molecules (H2O) as a single particle type.

Mixtures

A mixture contains two or more substances physically combined. These substances may be elements, compounds, or both.

AP Chemistry diagram of a mixture showing water molecules and ethanol molecules physically combined.

Key features of mixtures:

Example Air (a mixture of gases like N2, O2, CO2)

Elemental Composition by Mass

For both pure substances and mixtures, we can analyse elemental composition by mass — this means measuring the percentage of each element present in a sample.

For a pure substance, these percentages should match its chemical formula exactly.
For a mixture, the elemental composition will reflect the relative proportions of each substance present.

Using Elemental Analysis to Check Purity

Elemental analysis provides the mass percentages of elements in a sample. These values can be compared to the theoretical composition of a pure compound.

If the actual values differ significantly from the expected ones, the sample may be impure or contain other substances.

Worked Example

A sample of magnesium oxide (MgO) is tested and found to contain:
60.3% magnesium
39.7% oxygen

  1. Step 1: Calculate expected mass percentages for pure MgO
    Molar mass of MgO = 24.31 + 16.00 = 40.31 g/mol
    % Mg = (24.31 ÷ 40.31) × 100 = 60.3%
    % O = (16.00 ÷ 40.31) × 100 = 39.7%
  2. Step 2: Compare with measured values
    Measured values match expected values exactly → Sample is pure MgO.

Answer: The sample is pure MgO.


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Matt’s exam tip

Don’t assume all measured compositions are accurate. Even small differences can indicate impurities, especially in questions where you are asked to justify purity based on data.

Summary