PrepGo

AP Chemistry Practice Quiz: Mass Spectra of Elements

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

A mass spectrum of a single element sample provides a graphical representation of data. What does the height of each peak on the spectrum directly represent?

All Questions (9)

A mass spectrum of a single element sample provides a graphical representation of data. What does the height of each peak on the spectrum directly represent?

A) The mass of a single isotope.

B) The relative natural abundance of an isotope.

C) The average atomic mass of the element.

D) The number of isotopes of the element.

Correct Answer: B

The y-axis (peak height) in a mass spectrum corresponds to the relative abundance of each species detected. The x-axis represents the mass-to-charge ratio, which for singly charged ions is effectively the isotopic mass.

Analysis of a pure sample of an unknown element using mass spectrometry would allow for the determination of which of the following? I. The identity of the isotopes of the element. II. The relative abundance of each isotope. III. The average atomic mass of the element.

A) I only

B) III only

C) I and II only

D) I, II, and III

Correct Answer: D

A mass spectrum directly shows the masses of the isotopes (I) and their relative abundances (II). From this data, the average atomic mass can be calculated as a weighted average (III). Therefore, all three can be determined from the analysis.

An element has two naturally occurring isotopes. Isotope X has a mass of 10.0 amu and a relative abundance of 20%. Isotope Y has a mass of 11.0 amu and a relative abundance of 80%. What is the estimated average atomic mass of the element?

A) 10.2 amu

B) 10.5 amu

C) 10.8 amu

D) 11.0 amu

Correct Answer: C

The average atomic mass is the weighted average of the isotopic masses. It is calculated as: (mass_X * abundance_X) + (mass_Y * abundance_Y) = (10.0 amu * 0.20) + (11.0 amu * 0.80) = 2.0 amu + 8.8 amu = 10.8 amu.

The mass spectrum of an element shows two major peaks: one at 35 amu and one at 37 amu. If the average atomic mass of the element listed on the periodic table is 35.45 amu, what can be concluded about the relative abundances of the two isotopes?

A) The isotope with a mass of 35 amu is more abundant.

B) The isotope with a mass of 37 amu is more abundant.

C) The two isotopes have nearly equal abundances.

D) The element has another undiscovered isotope with a mass of approximately 36 amu.

Correct Answer: A

The average atomic mass is a weighted average. Since the average atomic mass (35.45 amu) is closer to 35 amu than to 37 amu, the isotope with a mass of 35 amu must be present in a greater abundance, pulling the average closer to its own mass.

A mass spectrum for a sample of a single element shows three distinct peaks. What does the presence of three peaks indicate?

A) The element has an average atomic mass of 3.

B) The sample is a mixture of three different elements.

C) The element exists as three naturally occurring isotopes.

D) The element has an atomic number of 3.

Correct Answer: C

In the mass spectrum of a single element, each peak corresponds to a different isotope of that element. Therefore, three peaks indicate the presence of three isotopes.

The mass spectrum of an unknown element is shown. The height of the peak at mass 63 is approximately three times the height of the peak at mass 65. Which of the following is the best estimate for the average atomic mass of the element?

A) 63.0 amu

B) 63.5 amu

C) 64.0 amu

D) 64.5 amu

Correct Answer: B

A 3:1 ratio of heights means the relative abundances are approximately 75% for the isotope at 63 amu and 25% for the isotope at 65 amu. The weighted average is calculated as (63 amu * 0.75) + (65 amu * 0.25) = 47.25 + 16.25 = 63.5 amu.

The quantitative relationship between the mass spectrum of an element and its isotopes relies on two key measurements represented on the graph's axes. The x-axis typically represents the isotopic mass, while the y-axis represents the:

A) atomic number.

B) number of neutrons.

C) relative natural abundance.

D) total mass of the sample.

Correct Answer: C

A mass spectrum plots the mass-to-charge ratio (which is effectively the isotopic mass for +1 ions) on the x-axis against the relative intensity or relative abundance on the y-axis. This allows for the identification of isotopes and their prevalence.

An element Z has two isotopes, Z-79 and Z-81. If the average atomic mass of Z is 79.90 amu, which of the following statements must be true?

A) Z-79 is slightly less abundant than Z-81.

B) Z-79 is slightly more abundant than Z-81.

C) The abundances of Z-79 and Z-81 are exactly equal.

D) Z-81 is significantly more abundant than Z-79.

Correct Answer: B

The average atomic mass (79.90 amu) is a weighted average of the isotopic masses (79 and 81 amu). The midpoint between the two masses is 80.00 amu. Since the average mass is 79.90 amu, which is closer to 79 than to 81, the Z-79 isotope must be more abundant.

An element has two isotopes, Isotope 1 (mass M1, fractional abundance f1) and Isotope 2 (mass M2, fractional abundance f2). Which expression correctly calculates the average atomic mass of the element?

A) (M1 + M2) / 2

B) (M1 × f1) + (M2 × f2)

C) (M1 × M2) / (f1 + f2)

D) (M1 / f1) + (M2 / f2)

Correct Answer: B

The average atomic mass is defined as the weighted average of the masses of its naturally occurring isotopes. The formula is the sum of the product of each isotope's mass and its fractional abundance: Σ(mass_i × fractional_abundance_i). For two isotopes, this simplifies to (M1 × f1) + (M2 × f2).