Q: Which of the following is the best explanation for why the atomic mass of carbon is closest to 12.0000 amu?

Because $$^{12}\text{C} is $$the most abundant isotope of carbon.

Q: Rubidium has two naturally occurring isotopes: $$^{85}\text{Rb} (72.2$$%) and $$^{87}\text{Rb} (27.8$$%). If a mass spectrum shows a peak at 85 amu with 100% intensity and a peak at 87 amu with 50% intensity, what does this indicate?

$$^{85}\text{Rb} is $$twice as abundant as $$^{87}\text{Rb}.$$

Q: Which of the following equations correctly calculates the weighted average atomic mass for an element with two isotopes, A and B?

$$\text{Atomic mass} = (\text{fraction}_A \times \text{mass}_A) + (\text{fraction}_B \times \text{mass}_B)$$

Question 1

Which of the following best describes the difference between a normal average and a weighted average, as used in calculating atomic mass?

Accepted Answer

A weighted average multiplies each value by its relative abundance before summing.

Question 2

A naturally occurring element has two isotopes: Isotope A (mass = 68.9255 amu, abundance = 60.11%) and Isotope B (mass = 70.9247 amu, abundance = 39.89%). What is the atomic mass of this element? Use the formula: $$	ext{Atomic mass} = (	ext{fraction}_A 	imes 	ext{mass}_A) + (	ext{fraction}_B 	imes 	ext{mass}_B)$$

Accepted Answer

$$69.72$$ amu

Question 3

Which of the following statements about isotopes is correct?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 4

Magnesium has three naturally occurring isotopes: $$^{24}	ext{Mg} ($$mass = 23.9850 amu, abundance = 78.99%), $$^{25}	ext{Mg} ($$mass = 24.9858 amu, abundance = 10.00%), and $$^{26}	ext{Mg} ($$mass = 25.9826 amu, abundance = 11.01%). Calculate the average atomic mass of magnesium.

Accepted Answer

$$24.3050$$ amu

Question 5

Which mass spectrum below best represents a naturally occurring sample of copper, which has two isotopes with masses 62.94 amu and 64.93 amu and an atomic mass of 63.55 amu?

Accepted Answer

Image B

Question 6

A mass spectrometer separates ions based on which property?

Accepted Answer

Their mass-to-charge ratio

Question 7

A sample of chlorine contains 75.77% $$^{35}	ext{Cl} ($$mass = 34.97 amu) and 24.23% $$^{37}	ext{Cl} ($$mass = 36.97 amu). Calculate the atomic mass of chlorine.

Accepted Answer

$$35.45$$ amu

Question 8

Which of the following is the best explanation for why the atomic mass of carbon is closest to 12.0000 amu?

Accepted Answer

Because $$^{12}	ext{C} is $$the most abundant isotope of carbon.

Question 9

Which of the following best describes the information provided by a mass spectrum?

Accepted Answer

The relative abundances and masses of isotopes in a sample

Question 10

Rubidium has two naturally occurring isotopes: $$^{85}	ext{Rb} (72.2$$%) and $$^{87}	ext{Rb} (27.8$$%). If a mass spectrum shows a peak at 85 amu with 100% intensity and a peak at 87 amu with 50% intensity, what does this indicate?

Accepted Answer

$$^{85}	ext{Rb} is $$twice as abundant as $$^{87}	ext{Rb}.$$

Question 11

Which of the following equations correctly calculates the weighted average atomic mass for an element with two isotopes, A and B?

Accepted Answer

$$	ext{Atomic mass} = (	ext{fraction}_A 	imes 	ext{mass}_A) + (	ext{fraction}_B 	imes 	ext{mass}_B)$$

Question 12

Why is the atomic mass of chlorine (35.45 amu) not a whole number?

Accepted Answer

Because it is a weighted average of the masses of its isotopes.

Question 13

Which of the following real-world applications relies on the concept of isotopic abundance and atomic mass?

Accepted Answer

Radiocarbon dating in archaeology

Question 14

If a mass spectrum for an unknown element shows two peaks of nearly equal intensity at masses 62.94 amu and 64.93 amu, what can you infer about the isotopic composition?

Accepted Answer

The element has two isotopes with nearly equal natural abundances.

Question 15

Which of the following best explains why the atomic mass of an element can be used to identify the relative abundance of its isotopes?

Accepted Answer

Because the atomic mass is a weighted average based on isotope abundance.

Atomic Mass, Isotopes, and Mass Spectrometry

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