Q: Which of the following is the correct formula for calculating the average atomic mass of an element with two isotopes?

$$\text{Average mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2)$$

Question 1

Which of the following best describes a significant figure in a measurement?

Accepted Answer

All digits you are sure of plus one estimated digit.

Question 2

You measure the mass of a sample three times and record 0.1 kg, 0.0 kg, and 0.1 kg on a bathroom scale, and 54.4 g, 54.5 g, and 54.3 g on a lab scale. What does this scenario best illustrate?

Accepted Answer

The importance of using the most precise instrument available.

Question 3

A student adds 18 L of water to 0.01315 L. What is the correct sum, expressed with the appropriate number of significant figures?

Accepted Answer

$$18.01\ \mathrm{L}$$

Question 4

When multiplying or dividing measured quantities, the result should be reported with:

Accepted Answer

The same number of significant figures as the measurement with the fewest significant figures.

Question 5

A car travels 278 miles using 11.70 gallons of gasoline. What is the gas mileage, expressed with the correct number of significant figures?

Accepted Answer

$$23.8\ \mathrm{mi/gal}$$

Question 6

Which of the following is the correct definition of an atomic mass unit (amu)?

Accepted Answer

One twelfth the mass of a carbon-12 atom.

Question 7

Chlorine has two naturally occurring isotopes: chlorine-35 (mass = 34.969 amu, abundance = 75.76%) and chlorine-37 (mass = 36.966 amu, abundance = 24.24%). What is the average atomic mass of chlorine?

Accepted Answer

$$35.45\ \mathrm{amu}$$

Question 8

Which statement best explains why the atomic weight of an element is not always a whole number?

Accepted Answer

Because it is a weighted average of all naturally occurring isotopes.

Question 9

How many atoms are in 1 mole of any substance?

Accepted Answer

$$6.022 	imes 10$$^{23}$$

Question 10

A graphite pencil tip weighs 15 mg. If the molar mass of carbon is $$12.01\ \mathrm{g/mol}$$, how many carbon atoms does the pencil tip contain?

Accepted Answer

$$7.52 	imes 10$$^{20}$$

Question 11

Which of the following best describes Avogadro's number?

Accepted Answer

The number of atoms in one mole of a substance.

Question 12

If 2.258 $$	imes$$ $$10^{22}$$ atoms of silicon are present, how many moles of silicon does this represent?

Accepted Answer

$$0.0375\ \mathrm{mol}$$

Question 13

Which of the following is the correct method for determining the number of significant figures in the measurement 0.00340 g?

Accepted Answer

Count all digits except leading zeros; 0.00340 has 3 significant figures.

Question 14

A sample contains 1.50 g of element X, and 2 atoms of X have a mass of 1.50 g. What is the mass of one atom of X in grams?

Accepted Answer

$$0.75\ \mathrm{g}$$

Question 15

Which of the following is NOT a base SI unit?

Accepted Answer

Gram

Question 16

Which conversion factor would you use to convert 5.0 kilometers to meters?

Accepted Answer

$$1\ \mathrm{km} = 1000\ \mathrm{m}$$

Question 17

Which of the following best explains why samples of helium, sulfur, copper, and mercury containing 1 mole each do not have the same mass?

Accepted Answer

Because each element has a different atomic mass.

Question 18

Which of the following is the correct formula for calculating the average atomic mass of an element with two isotopes?

Accepted Answer

$$	ext{Average mass} = (	ext{mass}_1 	imes 	ext{fraction}_1) + (	ext{mass}_2 	imes 	ext{fraction}_2)$$

Question 19

Which of the following is the correct value for the mass of 1 amu in grams?

Accepted Answer

$$1.6605 	imes 10$$^{-24}$$\ \mathrm{g}$$

The Math and Physics of Chemistry: Units, Measurement, and Atomic Mass

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