Q: Avogadro’s number is fundamental in chemistry. What is its value and what does it represent?

$$6.022 \times 10$$^{23}$$; the number of molecules or formula units in one mole of a substance.

Q: A prescription bottle contains 0.082 mol of ibuprofen. If the molar mass of ibuprofen is $$206.3 \text{ g/mol}$$, how many grams of ibuprofen are in the bottle?

$$0.082 \text{ mol} \times 206.3 \text{ g/mol} = 16.12 \text{ g}$$

Q: Which of the following is the correct formula for calculating percent yield in a chemical reaction?

$$\text{Percent yield} = \frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100$$

Q: In the reaction $$4\text{Fe}(s) + 3\text{O}_2(g) \rightarrow 2\text{Fe}_2\text{O}_3(s)$$, how many moles of $$\text{Fe}_2\text{O}_3$$ are produced from 6.2 mol of Fe?

$$6.2 \text{ mol Fe} \times \frac{2 \text{ mol Fe}_2\text{O}_3}{4 \text{ mol Fe}} = 3.1 \text{ mol Fe}_2\text{O}_3$$

Q: A reaction produced 0.022 g of calcium oxalate ($$\text{CaC}_2\text{O}_4). If $$the molar mass of $$\text{CaC}_2\text{O}_4 is$$ $$128.1 \text{ g/mol}$$ and the molar mass of $$\text{CaCl}_2 is$$ $$111.0 \text{ g/mol}$$, how many grams of $$\text{CaCl}_2$$ were used?

$$0.022 \text{ g CaC}_2\text{O}_4 \times \frac{1 \text{ mol CaC}_2\text{O}_4}{128.1 \text{ g CaC}_2\text{O}_4} \times \frac{1 \text{ mol CaCl}_2}{1 \text{ mol CaC}_2\text{O}_4} \times 111.0 \text{ g CaCl}_2 = 0.019 \text{ g CaCl}_2$$

Q: If the actual yield of $$\text{CO}_2 in a $$combustion reaction is 72.4 g and the theoretical yield is 88.0 g, what is the percent yield?

$$\frac{72.4}{88.0} \times 100 = 82.3\%$$

Q: In the reaction $$2\text{CH}_2(g) + 5\text{O}_2(g) \rightarrow 4\text{CO}_2(g) + 2\text{H}_2\text{O}(g)$$, what is the mole ratio of $$\text{O}_2 to$$ $$\text{CO}_2$$?

$$\frac{5 \text{ mol O}_2}{4 \text{ mol CO}_2}$$

Question 1

Which statement best describes the mole in chemistry?

Accepted Answer

A mole is the amount of a substance whose mass in grams is numerically equal to its molecular or formula weight.

Question 2

Avogadro’s number is fundamental in chemistry. What is its value and what does it represent?

Accepted Answer

$$6.022 	imes 10$$^{23}$$; the number of molecules or formula units in one mole of a substance.

Question 3

A prescription bottle contains 0.082 mol of ibuprofen. If the molar mass of ibuprofen is $$206.3 	ext{ g/mol}$$, how many grams of ibuprofen are in the bottle?

Accepted Answer

$$0.082 	ext{ mol} 	imes 206.3 	ext{ g/mol} = 16.12 	ext{ g}$$

Question 4

Which of the following is the correct formula for calculating percent yield in a chemical reaction?

Accepted Answer

$$	ext{Percent yield} = \frac{	ext{Actual yield}}{	ext{Theoretical yield}} 	imes 100$$

Question 5

In the reaction $$4	ext{Fe}(s) + 3	ext{O}_2(g) ightarrow 2	ext{Fe}_2	ext{O}_3(s)$$, how many moles of $$	ext{Fe}_2	ext{O}_3$$ are produced from 6.2 mol of Fe?

Accepted Answer

$$6.2 	ext{ mol Fe} 	imes \frac{2 	ext{ mol Fe}_2	ext{O}_3}{4 	ext{ mol Fe}} = 3.1 	ext{ mol Fe}_2	ext{O}_3$$

Question 6

A reaction produced 0.022 g of calcium oxalate ($$	ext{CaC}_2	ext{O}_4). If $$the molar mass of $$	ext{CaC}_2	ext{O}_4 is$$ $$128.1 	ext{ g/mol}$$ and the molar mass of $$	ext{CaCl}_2 is$$ $$111.0 	ext{ g/mol}$$, how many grams of $$	ext{CaCl}_2$$ were used?

Accepted Answer

$$0.022 	ext{ g CaC}_2	ext{O}_4 	imes \frac{1 	ext{ mol CaC}_2	ext{O}_4}{128.1 	ext{ g CaC}_2	ext{O}_4} 	imes \frac{1 	ext{ mol CaCl}_2}{1 	ext{ mol CaC}_2	ext{O}_4} 	imes 111.0 	ext{ g CaCl}_2 = 0.019 	ext{ g CaCl}_2$$

Question 7

If the actual yield of $$	ext{CO}_2 in a $$combustion reaction is 72.4 g and the theoretical yield is 88.0 g, what is the percent yield?

Accepted Answer

$$\frac{72.4}{88.0} 	imes 100 = 82.3\%$$

Question 8

Which of the following statements about balanced chemical equations is correct?

Accepted Answer

Balanced chemical equations must be read in terms of moles, not grams.

Question 9

In the reaction $$2	ext{CH}_2(g) + 5	ext{O}_2(g) ightarrow 4	ext{CO}_2(g) + 2	ext{H}_2	ext{O}(g)$$, what is the mole ratio of $$	ext{O}_2 to$$ $$	ext{CO}_2$$?

Accepted Answer

$$\frac{5 	ext{ mol O}_2}{4 	ext{ mol CO}_2}$$

Question 10

Which of the following is NOT a correct use of molar mass?

Accepted Answer

To convert between moles and molecules using Avogadro’s number.

Question 11

A student mixes 5.0 g of $$	ext{A}$$ and 10.0 g of $$	ext{B} in $$the reaction $$A + 2B ightarrow 2AB. If $$the molar masses are $$A = 5.0 	ext{ g/mol}$$ and $$B = 2.0 	ext{ g/mol}$$, which is the limiting reagent?

Accepted Answer

B is the limiting reagent.

Question 12

Which of the following best describes the difference between molecular weight and formula weight?

Accepted Answer

Molecular weight is the sum of atomic weights in a molecule; formula weight is the sum of atomic weights in one formula unit of any compound.

Question 13

A chemist wants to produce 10.0 g of $$	ext{NaCl}$$ from $$	ext{Na}_2	ext{CO}_3$$ and $$	ext{HCl}. If $$the molar mass of $$	ext{NaCl} is$$ $$58.44 	ext{ g/mol}$$, how many moles of $$	ext{NaCl}$$ does this represent?

Accepted Answer

$$\frac{10.0 	ext{ g}}{58.44 	ext{ g/mol}} = 0.171 	ext{ mol}$$

Question 14

Why is the actual yield of a chemical reaction often less than the theoretical yield?

Accepted Answer

Because of side reactions and loss of product during handling.

Question 15

Which conversion factor would you use to convert moles of $$	ext{H}_2 to $$moles of $$	ext{NH}_3 in $$the reaction $$3	ext{H}_2 + 	ext{N}_2 ightarrow 2	ext{NH}_3$$?

Accepted Answer

$$\frac{2 	ext{ mol NH}_3}{3 	ext{ mol H}_2}$$

Question 16

A sample contains $$3.01 	imes 10$$^{23}$$ molecules of water. How many moles of water does this represent?

Accepted Answer

$$\frac{3.01 	imes 10$$^{23}$$}{6.022 	imes 10$$^{23}$$} = 0.5 	ext{ mol}$$

Question 17

Which step is NOT required when performing a mass-to-mass stoichiometric calculation?

Accepted Answer

Directly converting grams of reactant to grams of product without using moles.

Question 18

In a reaction, the limiting reagent is:

Accepted Answer

The reactant that is exhausted first and limits the amount of product formed.

Chap 6: Chemical Reactions: Mole and Mass Relationships

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