1.4 Composition of Mixtures

01Mixtures vary

Unlike compounds, mixtures can have any ratio of components. Elemental analysis of a mixture tells you how much of each pure substance is present.

Typical AP question: a sample is part NaCl and part something else — use the mass of one element to back out the mass of the compound containing it.

02Notebook box

Mass % of component = (mass of component ÷ mass of mixture) × 100.

Worked example: A 10.0 g mixture yields 2.0 g Cl, all from NaCl (60.7% Cl). Mass NaCl = 2.0 ÷ 0.607 = 3.3 g → 33% NaCl.

Key takeaways

✦Mixtures have variable composition.
✦Track one element back to its compound.
✦Percent composition differs between samples of a mixture.

Quick check

Did it stick?

1.Which has a fixed composition?

2.5.0 g of a 20.0 g mixture is sugar. Mass % sugar?

3.Two samples of a mixture have different % Fe. This shows…