Q: A 16.2 g aluminum can is made of pure aluminum. How many aluminum atoms does it contain? (Molar mass of Al = $$26.98\ \mathrm{g/mol})$$

$$3.62 \times 10$$^{23}$$ atoms

Q: Which of the following is the correct formula to calculate the mass percent of an element X in a compound?

$$\text{mass percent of element X} = \frac{\text{mass of X in a sample of the compound}}{\text{mass of the sample of the compound}} \times 100\%$$

Q: Which of the following is the correct formula for calculating the number of molecules in a given mass of a compound?

$$\text{Number of molecules} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \times 6.022 \times 10$$^{23}$$

Question 1

A nutritionist wants to know how many grams of sodium chloride (NaCl) a person can consume and still stay below the FDA recommendation of 2.4 g sodium per day. Sodium chloride is 39% sodium by mass. What is the maximum mass of NaCl that can be consumed per day to stay within the FDA guideline?

Accepted Answer

$$6.2\ \mathrm{g}$$

Question 2

Which of the following statements best describes Avogadro’s number?

Accepted Answer

It is the number of atoms in exactly 12 g of pure carbon-12.

Question 3

How many atoms are present in $$0.58\ \mathrm{g} of $$diamond (pure carbon)? (Atomic mass of C = $$12.01\ \mathrm{g/mol})$$

Accepted Answer

$$2.9 	imes 10$$^{22}$$ atoms

Question 4

A sample contains $$1.1 	imes 10$$^{22}$$ silver atoms. How many moles of silver does this represent? (Avogadro’s number = 6.022$$ 	imes $$10^{23})$$

Accepted Answer

$$1.8 	imes 10$$^{-2}$$ mol

Question 5

What is the molar mass of $$\mathrm{NO_2$}$$?

Accepted Answer

$$46.01\ \mathrm{g/mol}$$

Question 6

How many moles of oxygen atoms are present in $$1.7\ \mathrm{mol} of$$ $$\mathrm{$$CaCO_3$$}$$?

Accepted Answer

$$5.1\ \mathrm{mol}$$

Question 7

A 16.2 g aluminum can is made of pure aluminum. How many aluminum atoms does it contain? (Molar mass of Al = $$26.98\ \mathrm{g/mol})$$

Accepted Answer

$$3.62 	imes 10$$^{23}$$ atoms

Question 8

What is the mass percent of chlorine in $$\mathrm{C_2Cl_4F_2} ($$freon-114)? (Molar mass Cl = $$35.45\ \mathrm{g/mol}$$, Molar mass $$\mathrm{C_2Cl_4F_2}$$ = $$203.8\ \mathrm{g/mol})$$

Accepted Answer

$$69.58\%$$

Question 9

A 0.358-g sample of chromium reacts with oxygen to form 0.523 g of the metal oxide. What is the mass percent of chromium in the oxide?

Accepted Answer

$$68.5\%$$

Question 10

A 3.24-g sample of titanium reacts with oxygen to form 5.40 g of the metal oxide. What is the empirical formula of the oxide? (Atomic mass Ti = $$47.87\ \mathrm{g/mol}$$, O = $$16.00\ \mathrm{g/mol})$$

Accepted Answer

$$\mathrm{$$TiO_2$$}$$

Question 11

A compound has an empirical formula of $$\mathrm{CH_2O}$$ and a molar mass of $$180.2\ \mathrm{g/mol}. $$What is its molecular formula?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$}$$

Question 12

Which of the following best describes the relationship between the empirical formula and the molecular formula of a compound?

Accepted Answer

The molecular formula is always a whole-number multiple of the empirical formula.

Question 13

What is the mass of $$1.75\ \mathrm{mol} of $$water ($$\mathrm{H_2O}$$)? (Molar mass $$\mathrm{H_2O}$$ = $$18.02\ \mathrm{g/mol})$$

Accepted Answer

$$31.5\ \mathrm{g}$$

Question 14

Which of the following is the correct formula to calculate the mass percent of an element X in a compound?

Accepted Answer

$$	ext{mass percent of element X} = \frac{	ext{mass of X in a sample of the compound}}{	ext{mass of the sample of the compound}} 	imes 100\%$$

Question 15

If a compound contains 3.0 g of hydrogen and 24 g of oxygen, what is the empirical formula of the compound? (Atomic masses: H = $$1.01\ \mathrm{g/mol}$$, O = $$16.00\ \mathrm{g/mol})$$

Accepted Answer

$$\mathrm{H_2O}$$

Question 16

Which of the following best explains why the mass of 1 mole of atoms is different for different elements?

Accepted Answer

Different elements have different atomic masses.

Question 17

A chemist needs to prepare 1500 L of water fluoridated at a level of 1.0 mg F$$^-$$ per liter. What mass of NaF (sodium fluoride) should be added? (Molar mass NaF = $$41.99\ \mathrm{g/mol}$$, atomic mass F = $$19.00\ \mathrm{g/mol})$$

Accepted Answer

$$3.3\ \mathrm{g}$$

Question 18

Which of the following is the correct formula for calculating the number of molecules in a given mass of a compound?

Accepted Answer

$$	ext{Number of molecules} = \frac{	ext{mass (g)}}{	ext{molar mass (g/mol)}} 	imes 6.022 	imes 10$$^{23}$$

Chapter 6: Chemical Composition – The Mole, Molar Mass, and Chemical Formulas

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