Q: Which statement best describes the mole concept in chemistry?

A mole is a specific number of particles, equal to $$6.022 \times 10$$^{23}$$.

Q: How many atoms are in 0.58 g of carbon? (Atomic mass of C = 12.01 g/mol)

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

Q: A 16.2 g aluminum can contains how many aluminum atoms? (Molar mass of Al = 26.98 g/mol)

$$16.2 \text{ g} \times \frac{1 \text{ mol}}{26.98 \text{ g}} \times \frac{6.022 \times 10$$^{23}$$ \text{ atoms}}{1 \text{ mol}} = 3.62 \times 10$$^{23}$$ atoms

Q: What is the mass percent composition of chlorine in C2Cl4F2? (Molar mass Cl = 35.45 g, Molar mass C2Cl4F2 = 203.8 g)

$$\frac{4 \times 35.45 \text{ g}}{203.8 \text{ g}} \times 100\% = 69.58\%$$

Q: If the FDA recommends less than 2.4 g of sodium per day, how many grams of sodium chloride (NaCl) can you consume and stay within the guideline? (NaCl is 39% sodium by mass)

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

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

$$\frac{0.358 \text{ g}}{0.523 \text{ g}} \times 100\% = 68.5\%$$

Q: How many moles of oxygen are in 1.7 mol of CaCO3?

$$1.7 \text{ mol CaCO}_3 \times \frac{3 \text{ mol O}}{1 \text{ mol CaCO}_3} = 5.1 \text{ mol O}$$

Q: Which equation correctly expresses the relationship for calculating mass percent composition from a chemical formula?

$$\text{mass percent of element X} = \frac{\text{mass of X in 1 mol of compound}}{\text{mass of 1 mol of compound}} \times 100\%$$

Q: How many grams of water are in 1.75 mol of H$$_2O$$? (Molar mass H$$_2O = 18.02 g/$$mol)

$$1.75 \text{ mol} \times 18.02 \text{ g/mol} = 31.5 \text{ g}$$

Q: What is the mass of 4.78 × 10$$^{24}$$ molecules of NO$$_2$$? (Molar mass NO$$_2 = 46.01 g/$$mol)

$$4.78 \times 10$$^{24}$$ \text{ molecules} \times \frac{1 \text{ mol}}{6.022 \times 10$$^{23}$$ \text{ molecules}} \times 46.01 \text{ g/mol} = 365 \text{ g}$$

Question 1

Which statement best describes the mole concept in chemistry?

Accepted Answer

A mole is a specific number of particles, equal to $$6.022 	imes 10$$^{23}$$.

Question 2

How many atoms are in 0.58 g of carbon? (Atomic mass of C = 12.01 g/mol)

Accepted Answer

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

Question 3

A 16.2 g aluminum can contains how many aluminum atoms? (Molar mass of Al = 26.98 g/mol)

Accepted Answer

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

Question 4

What is the mass percent composition of chlorine in C2Cl4F2? (Molar mass Cl = 35.45 g, Molar mass C2Cl4F2 = 203.8 g)

Accepted Answer

$$\frac{4 	imes 35.45 	ext{ g}}{203.8 	ext{ g}} 	imes 100\% = 69.58\%$$

Question 5

If the FDA recommends less than 2.4 g of sodium per day, how many grams of sodium chloride (NaCl) can you consume and stay within the guideline? (NaCl is 39% sodium by mass)

Accepted Answer

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

Question 6

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

Accepted Answer

$$\frac{0.358 	ext{ g}}{0.523 	ext{ g}} 	imes 100\% = 68.5\%$$

Question 7

How many moles of oxygen are in 1.7 mol of CaCO3?

Accepted Answer

$$1.7 	ext{ mol CaCO}_3 	imes \frac{3 	ext{ mol O}}{1 	ext{ mol CaCO}_3} = 5.1 	ext{ mol O}$$

Question 8

What is the empirical formula for a compound formed from 3.0 g H and 24 g O? (Atomic masses: H = 1.01 g/mol, O = 16.00 g/mol)

Accepted Answer

H$$_2O$$

Question 9

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

Accepted Answer

TiO$$_2$$

Question 10

What is the molecular formula for fructose if its empirical formula is CH$$_2O $$and its molar mass is 180.2 g/mol? (Empirical formula molar mass = 30.03 g/mol)

Accepted Answer

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

Question 11

Which equation correctly expresses the relationship for calculating mass percent composition from a chemical formula?

Accepted Answer

$$	ext{mass percent of element X} = \frac{	ext{mass of X in 1 mol of compound}}{	ext{mass of 1 mol of compound}} 	imes 100\%$$

Question 12

How many grams of water are in 1.75 mol of H$$_2O$$? (Molar mass H$$_2O = 18.02 g/$$mol)

Accepted Answer

$$1.75 	ext{ mol} 	imes 18.02 	ext{ g/mol} = 31.5 	ext{ g}$$

Question 13

What is the mass of 4.78 × 10$$^{24}$$ molecules of NO$$_2$$? (Molar mass NO$$_2 = 46.01 g/$$mol)

Accepted Answer

$$4.78 	imes 10$$^{24}$$ 	ext{ molecules} 	imes \frac{1 	ext{ mol}}{6.022 	imes 10$$^{23}$$ 	ext{ molecules}} 	imes 46.01 	ext{ g/mol} = 365 	ext{ g}$$

Question 14

Which of the following is NOT a correct use of Avogadro’s number?

Accepted Answer

To convert between grams and number of atoms directly.

Question 15

Which step is necessary when determining an empirical formula from mass percent composition?

Accepted Answer

Convert the mass of each element to moles using the molar mass.

Question 16

If a compound has an empirical formula of CH$$_2O $$and a molar mass of 90.09 g/mol, what is its molecular formula?

Accepted Answer

C$$_3H$_6O$_3$$

Question 17

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

Accepted Answer

Different elements have different atomic masses.

Chapter 6: Chemical Composition – Study Guide

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