Q: How many atoms are present in 0.58 g of diamond (pure carbon)? (Atomic mass of C = 12.01 g/mol, Avogadro's number = $$6.022 \times 10$$^{23}$$)

$$0.58\ \mathrm{g} \times \frac{1\ \mathrm{mol}}{12.01\ \mathrm{g}} \times 6.022 \times 10$$^{23}$$\ \mathrm{atoms/mol} = 2.91 \times 10$$^{22}$$\ \mathrm{atoms}$$

Q: Which of the following is the correct molar mass of nitrogen dioxide (NO$$_2$$)? (Atomic masses: N = 14.01 g/mol, O = 16.00 g/mol)

$$14.01 + 2 \times 16.00 = 46.01\ \mathrm{g/mol}$$

Q: A sample contains $$4.78 \times 10$$^{24}$$ molecules of NO_2$. What is the mass of this sample? (Molar mass NO_2$ = 46.01 g/mol, Avogadro's number = 6.022$$ \times $$10^{23})$$

$$\frac{4.78 \times 10$$^{24}$$}{6.022 \times 10$$^{23}$$} \times 46.01 = 365\ \mathrm{g}$$

Q: How many moles of oxygen atoms are present in 1.7 mol of CaCO$$_3$$?

$$1.7\ \mathrm{mol\ $$CaCO_3$$} \times \frac{3\ \mathrm{mol\ O}}{1\ \mathrm{mol\ $$CaCO_3$$}} = 5.1\ \mathrm{mol\ O}$$

Q: A 0.358-g sample of chromium forms 0.523 g of a metal oxide. What is the mass percent of chromium in the oxide?

$$\frac{0.358}{0.523} \times 100\% = 68.4\%$$

Q: Which of the following is the correct formula to calculate the mass percent of chlorine in CCl$$_2F$_2$$?

$$\frac{2 \times \text{molar mass Cl}}{\text{molar mass CCl}_2\text{F}_2} \times 100\%$$

Question 1

Which of the following best explains why chemists use the mole as a counting unit for atoms and molecules?

Accepted Answer

Atoms and molecules are too small and numerous to count individually, so the mole allows chemists to count by weighing.

Question 2

A nutrition label states that a serving of food contains 2.4 g of sodium. If sodium chloride (NaCl) is 39% sodium by mass, what is the maximum mass of NaCl that can be consumed to stay within the FDA guideline?

Accepted Answer

$$\frac{2.4\ \mathrm{g\ Na}}{0.39} = 6.15\ \mathrm{g\ NaCl}$$

Question 3

How many atoms are present in 0.58 g of diamond (pure carbon)? (Atomic mass of C = 12.01 g/mol, Avogadro's number = $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$0.58\ \mathrm{g} 	imes \frac{1\ \mathrm{mol}}{12.01\ \mathrm{g}} 	imes 6.022 	imes 10$$^{23}$$\ \mathrm{atoms/mol} = 2.91 	imes 10$$^{22}$$\ \mathrm{atoms}$$

Question 4

Which of the following is the correct molar mass of nitrogen dioxide (NO$$_2$$)? (Atomic masses: N = 14.01 g/mol, O = 16.00 g/mol)

Accepted Answer

$$14.01 + 2 	imes 16.00 = 46.01\ \mathrm{g/mol}$$

Question 5

A sample contains $$4.78 	imes 10$$^{24}$$ molecules of NO_2$. What is the mass of this sample? (Molar mass NO_2$ = 46.01 g/mol, Avogadro's number = 6.022$$ 	imes $$10^{23})$$

Accepted Answer

$$\frac{4.78 	imes 10$$^{24}$$}{6.022 	imes 10$$^{23}$$} 	imes 46.01 = 365\ \mathrm{g}$$

Question 6

Which of the following best describes the empirical formula?

Accepted Answer

The simplest whole-number ratio of atoms in a compound.

Question 7

Given the empirical formula CH$$_2O $$and a molar mass of 180.2 g/mol, what is the molecular formula of the compound?

Accepted Answer

C$$_6H$_{12}O$_6$$

Question 8

How many moles of oxygen atoms are present in 1.7 mol of CaCO$$_3$$?

Accepted Answer

$$1.7\ \mathrm{mol\ $$CaCO_3$$} 	imes \frac{3\ \mathrm{mol\ O}}{1\ \mathrm{mol\ $$CaCO_3$$}} = 5.1\ \mathrm{mol\ O}$$

Question 9

A 0.358-g sample of chromium forms 0.523 g of a metal oxide. What is the mass percent of chromium in the oxide?

Accepted Answer

$$\frac{0.358}{0.523} 	imes 100\% = 68.4\%$$

Question 10

Which of the following is the correct formula to calculate the mass percent of chlorine in CCl$$_2F$_2$$?

Accepted Answer

$$\frac{2 	imes 	ext{molar mass Cl}}{	ext{molar mass CCl}_2	ext{F}_2} 	imes 100\%$$

Question 11

If a compound has a mass percent composition of 69.58% Cl and a molar mass of 203.8 g/mol, what is the mass of Cl in 100 g of the compound?

Accepted Answer

69.58 g

Question 12

Which of the following steps is NOT required when determining an empirical formula from experimental mass data?

Accepted Answer

Multiplying all mole values by Avogadro's number.

Question 13

A can contains 16.2 g of aluminum (Al). How many atoms of Al are in the can? (Molar mass Al = 26.98 g/mol, Avogadro's number = $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$16.2\ \mathrm{g} 	imes \frac{1\ \mathrm{mol}}{26.98\ \mathrm{g}} 	imes 6.022 	imes 10$$^{23}$$\ \mathrm{atoms/mol} = 3.62 	imes 10$$^{23}$$\ \mathrm{atoms}$$

Question 14

Which of the following statements about molar mass is correct?

Accepted Answer

The molar mass of an element is numerically equal to its atomic mass in grams per mole.

Question 15

Which conversion factor would you use to determine the number of hydrogen atoms in 2.5 mol of H$$_2O$$?

Accepted Answer

$$2\ \mathrm{mol\ H} / 1\ \mathrm{mol\ H_2O}$$

Question 16

Which of the following best describes the relationship between mass, moles, and number of particles?

Accepted Answer

Mass can be converted to moles using molar mass, and moles can be converted to number of particles using Avogadro's number.

Question 17

If a compound's empirical formula is HO and its molar mass is 34.0 g/mol, what is its molecular formula? (Empirical formula mass = 17.0 g/mol)

Accepted Answer

H$$_2O$_2$$

Question 18

Which of the following is a correct use of mass percent as a conversion factor?

Accepted Answer

$$\frac{39\ \mathrm{g\ Na}}{100\ \mathrm{g\ NaCl}}$$

Chemical Composition, Moles, and Stoichiometry: Study Guide for Introductory Chemistry

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