AP Chemistry Unit 1 Study Notes

AP Chemistry 1.4: Composition of mixtures

Determine how much of each component is present in a mixture.

Aligned to Atomic Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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Overview 4.1 Pure Substance vs. Mixture 4.2 Mass Percent in a Mixture 4.3 Finding Component Mass 4.4 Elemental Analysis of a Mixture 4.5 Purity 4.6 Using Elemental Percent to Detect Impurities

Source note

These detailed Unit 1 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.

Overview Open

  • Khan Academy's Unit 1 emphasizes using elemental analysis to find how much of a substance is in a mixture and to determine whether a sample may contain impurities.

4.1 Pure Substance vs. Mixture Open

  • A pure substance contains one chemical substance with a fixed composition.

    • Examples:

    • pure NaCl

    • pure H₂O

    • pure O₂

  • A mixture contains more than one substance physically combined.

    • Examples:

    • air

    • salt water

    • soil

4.2 Mass Percent in a Mixture Open

  • Formula:

    • mass % = (mass of component / total mass of mixture) × 100

    • Example:

  • A 50.0 g sample contains 10.0 g NaCl.

  • Mass percent:

    • (10.0 / 50.0)(100)

    • = 20.0% NaCl

4.3 Finding Component Mass Open

  • If you know the percentage and total mass:

    • mass component = decimal mass fraction × total mass

    • Example:

  • A 200 g mixture contains 35.0% compound X.

    • 35.0% = 0.350

    • mass X:

    • 0.350 × 200

    • = 70.0 g

4.4 Elemental Analysis of a Mixture Open

  • Questions may not directly tell you the percentage of the compound.

    • Instead, they may tell you the percentage of an element contained inside that compound .

    • For example, Khan Academy uses the idea that if potassium in a supplement exists as KCl, the measured amount of K can be used to determine how much KCl is in the sample.

  • The reasoning is:

    • measured K → moles K → moles KCl → grams KCl

    • because each formula unit of KCl contains one potassium ion.

4.5 Purity Open

  • A real sample might contain contaminants.

  • Percent purity can be described as:

  • (mass of desired pure substance / total sample mass) × 100

  • If:

    • 10.0 g sample

    • 8.50 g desired compound

    • then:

    • (8.50 / 10.0)(100)

    • = 85.0% pure

4.6 Using Elemental Percent to Detect Impurities Open

  • A pure compound has a predictable percent composition.

    • For example, pure NaCl should have a specific percentage of chlorine by mass.

  • If your measured sample has a very different chlorine percentage, that suggests another substance may be mixed with it.

  • Khan Academy gives this kind of reasoning in its mixture-purity material.

    • AP reasoning

    • Suppose your expected compound has 60% chlorine.

  • The experimental sample has 70% chlorine.

  • A possible contaminant should usually contain a higher percentage of chlorine than the desired compound if it is responsible for increasing the measured chlorine percentage.

    • This is a conceptual question, not just a calculation.