Q: If you have 0.58 g of krypton (Kr), and the molar mass of Kr is $$83.80\ \mathrm{g/mol}$$, how many krypton atoms do you have?

$$0.58\ \mathrm{g} \times \frac{1\ \mathrm{mol}}{83.80\ \mathrm{g}} \times 6.022 \times 10$$^{23}$$\ \mathrm{atoms/mol} = 4.17 \times 10$$^{21}$$ atoms

Q: What is the molar mass of $$\mathrm{CO}_2$$?

$$12.01\ \mathrm{g/mol} + 2 \times 16.00\ \mathrm{g/mol} = 44.01\ \mathrm{g/mol}$$

Q: A sample contains $$2.4\ \mathrm{g} of Na. If $$the mass percent of Na in NaCl is $$39\%$$, what is the maximum mass of NaCl you can consume?

$$2.4\ \mathrm{g\ Na} \times \frac{100\ \mathrm{g\ NaCl}}{39\ \mathrm{g\ Na}} = 6.2\ \mathrm{g\ NaCl}$$

Q: Which equation correctly expresses the mass percent of an element in a compound?

$$\text{mass percent} = \frac{\text{total mass of element in 1 mole}}{\text{mass of compound in 1 mole}} \times 100$$

Q: How many moles of hydrogen atoms are present in $$2.0$$ moles of $$\mathrm{C}_2\mathrm{H}_6$$?

$$2.0\ \mathrm{mol}\ \mathrm{C}_2\mathrm{H}_6 \times \frac{6\ \mathrm{mol\ H}}{1\ \mathrm{mol}\ \mathrm{C}_2\mathrm{H}_6} = 12\ \mathrm{mol\ H}$$

Q: You buy $$2.60$$ pounds of nails at the hardware store. If nails weigh $$0.150 lb/$$dozen, how many nails did you buy?

$$2.60\ \mathrm{lb} \times \frac{1\ \mathrm{dozen}}{0.150\ \mathrm{lb}} \times 12 = 208$$ nails

Question 1

Which of the following best defines a mole in chemistry?

Accepted Answer

The number of atoms in 12 grams of pure carbon-12.

Question 2

If you have 0.58 g of krypton (Kr), and the molar mass of Kr is $$83.80\ \mathrm{g/mol}$$, how many krypton atoms do you have?

Accepted Answer

$$0.58\ \mathrm{g} 	imes \frac{1\ \mathrm{mol}}{83.80\ \mathrm{g}} 	imes 6.022 	imes 10$$^{23}$$\ \mathrm{atoms/mol} = 4.17 	imes 10$$^{21}$$ atoms

Question 3

What is the molar mass of $$\mathrm{CO}_2$$?

Accepted Answer

$$12.01\ \mathrm{g/mol} + 2 	imes 16.00\ \mathrm{g/mol} = 44.01\ \mathrm{g/mol}$$

Question 4

A sample contains $$2.4\ \mathrm{g} of Na. If $$the mass percent of Na in NaCl is $$39\%$$, what is the maximum mass of NaCl you can consume?

Accepted Answer

$$2.4\ \mathrm{g\ Na} 	imes \frac{100\ \mathrm{g\ NaCl}}{39\ \mathrm{g\ Na}} = 6.2\ \mathrm{g\ NaCl}$$

Question 5

Which equation correctly expresses the mass percent of an element in a compound?

Accepted Answer

$$	ext{mass percent} = \frac{	ext{total mass of element in 1 mole}}{	ext{mass of compound in 1 mole}} 	imes 100$$

Question 6

A compound contains $$60.00\% C$$, $$4.48\% H$$, and $$35.53\% O by $$mass. What is the empirical formula?

Accepted Answer

$$\mathrm{C}_5\mathrm{H}_4\mathrm{O}_2$$

Question 7

If the empirical formula of a compound is $$\mathrm{CH}_2\mathrm{O}$$ and its molar mass is $$180\ \mathrm{g/mol}$$, what is its molecular formula?

Accepted Answer

$$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$$

Question 8

How many moles of hydrogen atoms are present in $$2.0$$ moles of $$\mathrm{C}_2\mathrm{H}_6$$?

Accepted Answer

$$2.0\ \mathrm{mol}\ \mathrm{C}_2\mathrm{H}_6 	imes \frac{6\ \mathrm{mol\ H}}{1\ \mathrm{mol}\ \mathrm{C}_2\mathrm{H}_6} = 12\ \mathrm{mol\ H}$$

Question 9

A sample of $$27\ \mathrm{g} of$$ $$\mathrm{H}_2\mathrm{O} is $$decomposed into $$3.0\ \mathrm{g} of $$hydrogen and $$24\ \mathrm{g} of $$oxygen. What is the empirical formula of the compound?

Accepted Answer

$$\mathrm{H}_2\mathrm{O}$$

Question 10

Which of the following statements about molar mass is correct?

Accepted Answer

The molar mass in grams is numerically equivalent to the atomic mass in amu.

Question 11

You buy $$2.60$$ pounds of nails at the hardware store. If nails weigh $$0.150 lb/$$dozen, how many nails did you buy?

Accepted Answer

$$2.60\ \mathrm{lb} 	imes \frac{1\ \mathrm{dozen}}{0.150\ \mathrm{lb}} 	imes 12 = 208$$ nails

Question 12

Which conversion factor would you use to convert from grams of a compound to moles of a compound?

Accepted Answer

$$\frac{1\ \mathrm{mol}}{	ext{molar mass in g}}$$

Question 13

A chemical formula can be used as a conversion factor to determine:

Accepted Answer

The number of moles of an element in a compound.

Question 14

If a compound has a mass percent composition of $$40.0\% C$$, $$6.7\% H$$, and $$53.3\% O$$, what is its empirical formula?

Accepted Answer

$$\mathrm{CH}_2\mathrm{O}$$

Question 15

Which of the following is the correct sequence for determining an empirical formula from mass percent data?

Accepted Answer

Convert mass percent to grams, convert grams to moles, write a pseudoformula, divide by the smallest number of moles, adjust to whole numbers.

Chapter 6: Chemical Composition – Counting Atoms, Molecules, and Determining Formulas

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