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AP Chemistry Practice Quiz: Elemental Composition of Pure Substances

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

Test your understanding with short quizzes. This quiz has 10 questions to check your progress.

Question 1 of 10

The observation that a 10-gram sample of pure sodium chloride (table salt) and a 100-gram sample of pure sodium chloride both contain 39.3% sodium by mass is best explained by which of the following?

All Questions (10)

The observation that a 10-gram sample of pure sodium chloride (table salt) and a 100-gram sample of pure sodium chloride both contain 39.3% sodium by mass is best explained by which of the following?

A) The definition of an empirical formula

B) The law of definite proportions

C) The composition of individual molecules

D) The definition of a pure substance

Correct Answer: B

The law of definite proportions states that the mass ratio (and therefore mass percentage) of constituent elements in any pure sample of a compound is always the same, regardless of the sample's size.

Which of the following best defines the empirical formula of a compound?

A) A formula showing the total mass of each element in a sample.

B) A formula representing the exact number of atoms in a single molecule.

C) The chemical formula that lists the lowest whole-number ratio of atoms of the elements.

D) The formula describing the arrangement of ions in a crystal lattice.

Correct Answer: C

The provided content explicitly states that 'The empirical formula is the chemical formula that lists the lowest whole-number ratio of atoms of the elements in a compound.'

A pure substance is described as being composed of atoms or ions held in fixed proportions. This concept of 'fixed proportions' is most directly represented by which of the following?

A) The total mass of the substance

B) The physical state of the substance

C) The empirical formula of the substance

D) The number of individual molecules in a sample

Correct Answer: C

The empirical formula provides the lowest whole-number ratio of atoms, which is a quantitative description of the fixed proportions of elements within the pure substance mentioned in the definition.

According to the law of definite proportions, if a pure sample of a compound is found to contain 5 grams of element X and 10 grams of element Y, what will be the mass ratio of X to Y in a different, larger pure sample of the same compound?

A) The ratio will be larger because the sample is larger.

B) The ratio will be 1:2.

C) The ratio will be 2:1.

D) The ratio cannot be determined without knowing the total mass of the new sample.

Correct Answer: B

The law of definite proportions states that the mass ratio of constituent elements in any pure sample of a compound is always the same. The initial ratio is 5g X to 10g Y, which simplifies to 1:2. This ratio will be constant for any sample of the compound.

The quantitative relationship between the elemental composition by mass of a pure substance and its empirical formula implies that one can determine the empirical formula from which of the following pieces of information?

A) The total mass of the sample

B) The volume and density of the sample

C) The mass percentage of each element in the substance

D) The melting point of the substance

Correct Answer: C

The content states there is a quantitative relationship between elemental composition by mass and the empirical formula. This relationship allows chemists to use mass data, such as the mass percentage of each element, to calculate the lowest whole-number ratio of atoms (the empirical formula).

A substance is identified as a pure substance. Based on the provided information, what must be true about its fundamental composition?

A) It is composed of only one type of element.

B) It is composed of atoms or ions held in fixed proportions.

C) It must be a gas at standard temperature and pressure.

D) It must be composed of individual molecules, not ions.

Correct Answer: B

The content defines pure substances as being 'composed of individual molecules, or atoms/ions held in fixed proportions as described by a formula unit.' This is the key defining characteristic provided.

A particular compound has a molecular formula that shows the exact number of atoms in a single molecule. How does the empirical formula for this compound compare?

A) The empirical formula is always more complex than the molecular formula.

B) The empirical formula represents the mass ratio, while the molecular formula represents the atom ratio.

C) The empirical formula is the simplified, lowest whole-number ratio of the atoms shown in the molecular formula.

D) The empirical formula and molecular formula are unrelated concepts.

Correct Answer: C

The definition of the empirical formula is 'the chemical formula that lists the lowest whole-number ratio of atoms of the elements in a compound.' This is, by definition, a simplified version of the actual molecular formula (unless the molecular formula is already in its simplest ratio).

The law of definite proportions provides the essential principle that makes it possible to consistently determine a compound's unique...

A) total sample mass.

B) physical properties.

C) empirical formula.

D) reaction rate.

Correct Answer: C

Because the mass ratio of elements in a compound is always the same (Law of Definite Proportions), we can reliably use this mass data from any pure sample to calculate the consistent, lowest whole-number ratio of atoms, which is the empirical formula. This links the law to the formula.

Two different-sized, pure samples of an unknown compound are analyzed. Which of the following quantities MUST be identical for both samples?

A) The total number of formula units in each sample.

B) The volume of each sample.

C) The total mass of each sample.

D) The mass ratio of the constituent elements in each sample.

Correct Answer: D

This is a direct application of the law of definite proportions, which states that 'the mass ratio of constituent elements in any pure sample of a compound is always the same.' The other properties (total formula units, volume, total mass) are extensive properties that depend on the size of the sample.

The use of a 'formula unit' to describe the composition of a pure substance primarily emphasizes which core concept?

A) All pure substances are composed of individual, separate molecules.

B) The elements within a pure substance are present in fixed, definite proportions.

C) The mass of a substance is directly proportional to the number of its atoms.

D) Pure substances can only exist as extended crystalline solids.

Correct Answer: B

The content states that pure substances are composed of atoms/ions 'held in fixed proportions as described by a formula unit.' A formula unit, like an empirical formula, represents the simplest whole-number ratio, thus emphasizing the concept of fixed proportions.