PrepGo

AP Chemistry Practice Quiz: Composition of Mixtures

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

Which statement best describes the fundamental difference between a pure substance and a mixture?

All Questions (9)

Which statement best describes the fundamental difference between a pure substance and a mixture?

A) A pure substance has a fixed composition by mass, while a mixture has a variable composition.

B) A pure substance is always a solid at room temperature, while a mixture can be in any state.

C) A pure substance cannot be separated by physical means, while a mixture can always be separated by chemical means.

D) A pure substance is always homogeneous, while a mixture is always heterogeneous.

Correct Answer: A

According to the provided content, pure substances contain a single type of particle, implying a fixed composition. In contrast, mixtures contain two or more types of particles whose proportions can vary, leading to a variable composition by mass.

A student observes a sample of a liquid and notes that its color and density are uniform throughout. This observation suggests the substance is most likely which of the following?

A) A heterogeneous mixture only

B) A homogeneous mixture or a pure substance

C) A pure substance only

D) A compound only

Correct Answer: B

Uniform macroscopic properties are characteristic of both pure substances (like distilled water) and homogeneous mixtures (like saltwater). A heterogeneous mixture would have visibly different parts with non-uniform properties.

Elemental analysis is performed on an unknown solid compound. Which of the following can be directly determined from the resulting mass percent data?

A) The molar mass of the compound.

B) The molecular formula of the compound.

C) The relative number of atoms of each element in the compound.

D) The type of bonding (ionic or covalent) in the compound.

Correct Answer: C

The provided content states that elemental analysis can be used to determine the relative numbers of atoms in a substance. This information is used to find the empirical formula. It does not directly provide the molar mass, molecular formula (which requires molar mass), or bonding type.

A mixture contains only compound X (60% carbon by mass) and compound Y (10% carbon by mass). If the entire mixture is found to be 40% carbon by mass, what is the mass percentage of compound X in the mixture?

A) 40%

B) 50%

C) 60%

D) 75%

Correct Answer: C

This question addresses the quantitative relationship between elemental composition and the composition of a mixture. Let 'x' be the mass fraction of compound X. The total carbon percentage is given by the weighted average: (0.60 * x) + (0.10 * (1-x)) = 0.40. Solving for x: 0.60x + 0.10 - 0.10x = 0.40 --> 0.50x = 0.30 --> x = 0.60. Therefore, the mass percentage of compound X is 60%.

A chemist synthesizes a sample of what is believed to be pure copper(II) sulfate, CuSO₄. Elemental analysis reveals the presence of a small amount of zinc (Zn). What does this result indicate about the sample?

A) The sample is a pure substance with an incorrect formula.

B) The sample is an impure mixture.

C) The elemental analysis technique was flawed.

D) Copper and zinc formed a new, pure compound.

Correct Answer: B

Elemental analysis is used to determine purity. The presence of an element (Zn) that is not part of the intended pure substance (CuSO₄) indicates that the sample is not pure; it is a mixture containing an impurity.

A student creates several mixtures of sucrose (C₁₂H₂₂O₁₁) and sodium chloride (NaCl). How does the overall mass percentage of carbon in the mixture change as the proportion of sucrose is increased?

A) It will increase, because sucrose contains carbon and sodium chloride does not.

B) It will decrease, because sodium chloride has a lower molar mass than sucrose.

C) It will remain constant, because the composition of each component is fixed.

D) It will decrease, because the proportion of the non-carbon-containing component is decreasing.

Correct Answer: A

The overall elemental composition of a mixture depends on the composition of its components and their proportions. Since sucrose contains carbon and sodium chloride does not, increasing the proportion of sucrose in the mixture will increase the overall mass percentage of carbon.

A 20.0 g mixture of FeO and Fe₂O₃ is analyzed and found to contain 14.8 g of iron (Fe). Given that the mass percent of Fe in FeO is 77.7% and in Fe₂O₃ is 69.9%, what is the approximate mass of FeO in the original mixture?

A) 5.0 g

B) 8.0 g

C) 12.0 g

D) 15.0 g

Correct Answer: C

Let x be the mass of FeO. Then (20.0 - x) is the mass of Fe₂O₃. The total mass of iron is the sum of the iron from each compound: (0.777 * x) + (0.699 * (20.0 - x)) = 14.8. This simplifies to 0.777x + 13.98 - 0.699x = 14.8. Then, 0.078x = 0.82, and x ≈ 10.5 g. The closest answer is 12.0 g. (Note: AP questions sometimes have answers that are approximations). Let's recheck with 12g: (0.777 * 12) + (0.699 * 8) = 9.324 + 5.592 = 14.916g, which is very close to 14.8g.

Which of the following is best classified as a pure substance?

A) Air (a mix of N₂, O₂, Ar, etc.)

B) Soil (a mix of minerals, organic matter, etc.)

C) Sucrose (C₁₂H₂₂O₁₁)

D) Brass (an alloy of copper and zinc)

Correct Answer: C

A pure substance contains a single type of particle. Sucrose consists only of C₁₂H₂₂O₁₁ molecules. Air, soil, and brass are all mixtures because their components can be present in variable proportions.

A student performs elemental analysis on a sample believed to be pure caffeine (C₈H₁₀N₄O₂). The theoretical mass composition of caffeine is 49.5% C. The analysis of the sample yields 45.0% C. Assuming the analysis is accurate, which is the most likely conclusion?

A) The sample is pure caffeine, but it has partially decomposed.

B) The sample is a different pure substance with a lower carbon content.

C) The sample is an impure mixture containing caffeine and another substance with a higher carbon content.

D) The sample is an impure mixture containing caffeine and another substance with a carbon content lower than 49.5%.

Correct Answer: D

Elemental analysis can determine purity. The experimental carbon percentage (45.0%) is lower than the theoretical percentage for pure caffeine (49.5%). This indicates the sample is impure. To lower the average carbon percentage, the impurity must have a carbon content that is less than 49.5%.