Q: The atomic mass of an element is calculated as:

$$\text{Atomic Mass} = \sum (\text{Isotope Mass} \times \text{Fractional Abundance})$$

Q: Which of the following is the correct conversion between atomic mass units and kilograms?

$$1\ \text{amu} = 1.66 \times 10$$^{-27}$$\ \text{kg}$$

Q: A hospital uses a radioactive isotope with a mass of $$2.5 \times 10^2$ amu. What is this mass in kilograms?

$$4.15 \times 10$$^{-25}$$\ \text{kg}$$

Question 1

Which of the following units is NOT commonly used to express atomic mass?

Accepted Answer

kg

Question 2

The atomic mass of an element is calculated as:

Accepted Answer

$$	ext{Atomic Mass} = \sum (	ext{Isotope Mass} 	imes 	ext{Fractional Abundance})$$

Question 3

Which element from the first column of the periodic table has the greatest atomic mass?

Accepted Answer

Cs

Question 4

If an element has two isotopes with masses of 10 amu and 12 amu, and their percent abundances are 75% and 25% respectively, what is the atomic mass of the element?

Accepted Answer

$$10.5\ 	ext{amu}$$

Question 5

A sample contains three isotopes: 20% of isotope A (mass = 50 amu), 30% of isotope B (mass = 52 amu), and 50% of isotope C (mass = 54 amu). What is the atomic mass of the sample?

Accepted Answer

$$52.8\ 	ext{amu}$$

Question 6

Which of the following choices has the greatest atomic mass?

Accepted Answer

Element C (0.350 kg)

Question 7

Gallium has two naturally occurring isotopes: Ga-69 (mass = 68.9256 amu, abundance = 60.11%) and Ga-71 (mass = 70.9247 amu, abundance = 39.89%). What is the atomic mass of gallium?

Accepted Answer

$$69.72\ 	ext{amu}$$

Question 8

If the percent abundance of an isotope is given as 25%, what is its fractional abundance?

Accepted Answer

$$0.25$$

Question 9

Three isotopes of magnesium exist: $$^{24}	ext{Mg} ($$mass = 23.98504 amu, abundance = 78.70%), $$^{25}	ext{Mg} ($$mass = x amu, abundance = 10.13%), and $$^{26}	ext{Mg} ($$mass = 25.98259 amu, abundance = 11.17%). If the atomic mass of magnesium is 24.305 amu, what is the mass of $$^{25}	ext{Mg}$$?

Accepted Answer

$$24.99942\ 	ext{amu}$$

Question 10

Silver has an atomic mass of 107.868 amu. The Ag-109 isotope (108.905 amu) is 48.16% abundant. What is the amu of the other isotope?

Accepted Answer

$$106.905\ 	ext{amu}$$

Question 11

Which of the following statements about atomic mass is TRUE?

Accepted Answer

Atomic mass is the weighted average of all isotopes of an element.

Question 12

A chemist finds a new element with two isotopes: 40% of isotope X (mass = 100 amu) and 60% of isotope Y (mass = 110 amu). What is the atomic mass of the element?

Accepted Answer

$$106.5\ 	ext{amu}$$

Question 13

Which of the following best describes the term 'percent abundance'?

Accepted Answer

The percentage of an element's atoms that are a particular isotope.

Question 14

If an element has three isotopes with fractional abundances of 0.2, 0.3, and 0.5, what is the sum of their percent abundances?

Accepted Answer

$$100\%$$

Question 15

Which of the following is the correct conversion between atomic mass units and kilograms?

Accepted Answer

$$1\ 	ext{amu} = 1.66 	imes 10$$^{-27}$$\ 	ext{kg}$$

Question 16

A hospital uses a radioactive isotope with a mass of $$2.5 	imes 10^2$ amu. What is this mass in kilograms?

Accepted Answer

$$4.15 	imes 10$$^{-25}$$\ 	ext{kg}$$

Atomic Masses and Isotopic Calculations in GOB Chemistry

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