Q: Which equation correctly calculates the atomic mass ($$M_{avg}) of an $$element with two isotopes of masses $$m_1$$ and $$m_2$$ and abundances $$a_1$$ and $$a_2 ($$expressed as decimals)?

$$M_{avg} = (m_1$ \times a_1$) + (m_2$ \times a_2$)$$

Q: Which of the following is the correct formula for calculating a weighted average?

$$\text{Weighted Average} = \frac{\sum (\text{value} \times \text{weight})}{\sum \text{weights}}$$

Question 1

Which of the following best describes the difference between a simple average and a weighted average?

Accepted Answer

A simple average treats all values equally, while a weighted average assigns different levels of importance (weights) to each value.

Question 2

Which equation correctly calculates the atomic mass ($$M_{avg}) of an $$element with two isotopes of masses $$m_1$$ and $$m_2$$ and abundances $$a_1$$ and $$a_2 ($$expressed as decimals)?

Accepted Answer

$$M_{avg} = (m_1$ 	imes a_1$) + (m_2$ 	imes a_2$)$$

Question 3

Chlorine has two isotopes: one with a mass of 34.97 amu (75.77% abundance) and one with a mass of 36.97 amu (24.23% abundance). What is the atomic mass of chlorine? (Round to two decimal places)

Accepted Answer

$$35.45$$ amu

Question 4

A naturally occurring element has two isotopes: one with a mass of 68.9255 amu (60.11% abundance) and one with a mass of 70.9247 amu. What is the atomic mass of the element? (Round to two decimal places)

Accepted Answer

$$69.72$$ amu

Question 5

An element has two isotopes with masses of 34.9701 amu and 36.9702 amu. If its average atomic mass is 35.27 amu, what is the approximate abundance of the lighter isotope?

Accepted Answer

$$85\%$$

Question 6

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 7

Which mass spectrum (A, B, C, or D) 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

Spectrum B

Question 8

A mass spectrometer separates ions based on which property?

Accepted Answer

Their mass-to-charge ratio

Question 9

Rubidium has two isotopes: Rb-85 (72.2% abundance) and Rb-87 (27.8% abundance). Which of the following best explains why the atomic mass of rubidium is closer to 85 amu than 87 amu?

Accepted Answer

Because Rb-85 is more abundant, so it contributes more to the weighted average.

Question 10

Which of the following is closest to the atomic mass of carbon, given its isotopes: C-12 (98.93%, 12.0000 amu), C-13 (1.07%, 13.0034 amu), and C-14 (trace, 14.0032 amu)?

Accepted Answer

$$12.0000$$ amu

Question 11

Which of the following best describes what the height of a peak in a mass spectrum represents?

Accepted Answer

The relative abundance of the isotope

Question 12

If a mass spectrum shows three peaks for an element, what does this indicate?

Accepted Answer

The element has three naturally occurring isotopes.

Question 13

Which of the following is the correct formula for calculating a weighted average?

Accepted Answer

$$	ext{Weighted Average} = \frac{\sum (	ext{value} 	imes 	ext{weight})}{\sum 	ext{weights}}$$

Question 14

A student calculates the atomic mass of an element with two isotopes as $$M_{avg} = (m_1$ 	imes a_1$) + (m_2$ 	imes a_2$). If$$ $$m_1 = 10$ amu, a_1$ = 0.7$$, $$m_2 = 12$ amu, and a_2$ = 0.3$$, what is $$M_{avg}$$?

Accepted Answer

$$10.6$$\ amu

Question 15

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

Accepted Answer

Radiocarbon dating of archaeological artifacts

Question 16

If the abundance of the heavier isotope of an element increases, what happens to the atomic mass of the element?

Accepted Answer

It increases.

Question 17

Which of the following best explains why the atomic mass of an element is rarely a whole number?

Accepted Answer

Because atomic mass is a weighted average of the masses of all naturally occurring isotopes.

Atomic Mass, Isotopes, and Mass Spectrometry: Study Notes for General Chemistry

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