Question 1

Which of the following best describes the purpose of the equation $\mathrm{AM} = f_1M_1 + f_2M_2 + \ldots + f_nM_n$?

Accepted Answer

To determine the average atomic mass of an element based on the masses and abundances of its isotopes.

Question 2

A sample of chlorine contains two isotopes: $^{35}\mathrm{Cl}$ (mass = $34.97\ \mathrm{amu}$, abundance = $75.77\%$) and $^{37}\mathrm{Cl}$ (mass = $36.97\ \mathrm{amu}$, abundance = $24.23\%$). What is the average atomic mass of chlorine? Use the formula $\mathrm{AM} = f_1M_1 + f_2M_2$.

Accepted Answer

$35.45\ \mathrm{amu}$

Question 3

If an element has three naturally occurring isotopes with the following data: $f_1 = 0.50$, $M_1 = 10\ \mathrm{amu}$; $f_2 = 0.30$, $M_2 = 11\ \mathrm{amu}$; $f_3 = 0.20$, $M_3 = 12\ \mathrm{amu}$, what is the average atomic mass?

Accepted Answer

$10.7\ \mathrm{amu}$

Question 4

In the formula $\mathrm{AM} = f_1M_1 + f_2M_2 + \ldots + f_nM_n$, what does $f_n$ represent?

Accepted Answer

The fractional abundance of the $n$th isotope.

Question 5

A sample of magnesium contains $79.0\%$ $^{24}\mathrm{Mg}$ (mass = $23.99\ \mathrm{amu}$), $10.0\%$ $^{25}\mathrm{Mg}$ (mass = $24.99\ \mathrm{amu}$), and $11.0\%$ $^{26}\mathrm{Mg}$ (mass = $25.98\ \mathrm{amu}$). What is the average atomic mass of magnesium?

Accepted Answer

$24.31\ \mathrm{amu}$

Question 6

Why must the fractional abundances ($f_1, f_2, \ldots, f_n$) in the average atomic mass formula always add up to 1?

Accepted Answer

Because they represent the fraction of each isotope in a sample, and all fractions together must account for the whole sample.

Question 7

If an element has two isotopes, $A$ and $B$, with $f_A = 0.60$, $M_A = 20\ \mathrm{amu}$, and $f_B = 0.40$, $M_B = 22\ \mathrm{amu}$, what is the average atomic mass?

Accepted Answer

$20.8\ \mathrm{amu}$

Question 8

Which of the following statements is true regarding the calculation of average atomic mass?

Accepted Answer

The average atomic mass is a weighted average based on the relative abundances and masses of all isotopes.

Question 9

A student calculates the average atomic mass of an element with two isotopes as $0.25 	imes 10\ \mathrm{amu} + 0.75 	imes 12\ \mathrm{amu} = 11.5\ \mathrm{amu}$. What mistake, if any, did the student make?

Accepted Answer

No mistake; the calculation is correct.

Question 10

If the average atomic mass of boron is $10.81\ \mathrm{amu}$, and it has two isotopes, $^{10}\mathrm{B}$ (mass = $10.01\ \mathrm{amu}$) and $^{11}\mathrm{B}$ (mass = $11.01\ \mathrm{amu}$), which isotope is more abundant?

Accepted Answer

$^{11}\mathrm{B}$

CHEM 170 Exam 1 Study Guide: Foundations of Chemistry

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