Q: Given the balanced equation for the combustion of octane: $$2\ \mathrm{C_8H_{18}}(l) + 25\ \mathrm{O_2$}(g) \rightarrow 16\ \mathrm{CO_2$}(g) + 18\ \mathrm{H_2O}(g)$$, how many moles of $$\mathrm{CO_2$}$$ are produced from 5 moles of $$\mathrm{C_8H_{18}}$$?

$$16\ \mathrm{mol\ CO_2$} \times \frac{5}{2} = 40\ \mathrm{mol\ CO_2$}$$

Q: A car burns $$1.00 \times 10^4$ g of octane ($$\mathrm{C_8H_{18}}$$). Using the molar mass 114.3$$\ \mathrm{g/mol}$$ and the balanced equation 2$$\ \mathrm{C_8H_{18}} + 25\ \mathrm{$$O_2$$} \rightarrow 16\ \mathrm{$$CO_2$$} + 18\ \mathrm{H_2O}$$, how many grams of $$\mathrm{$$CO_2$$}$$ are produced?

$$1.00 \times 10^4$\ \mathrm{g\ C_8H_{18}} \times \frac{1\ \mathrm{mol}}{114.3\ \mathrm{g}} \times \frac{16\ \mathrm{mol\ CO_2$}}{2\ \mathrm{mol\ C_8H_{18}}} \times 44.01\ \mathrm{g\ CO_2$}/\mathrm{mol} = 3.08 \times 10^4$\ \mathrm{g\ CO_2$}$$

Q: A reaction between $$11.5\ \mathrm{g}$$ $$\mathrm{Cu_2O}$$ and $$114.5\ \mathrm{g} C $$produces $$87.4\ \mathrm{g} Cu. If $$the theoretical yield is $$101.7\ \mathrm{g} Cu$$, what is the percent yield?

$$\frac{87.4}{101.7} \times 100\% = 85.9\%$$

Q: How much heat (in kJ) is released when $$1.18 \times 10^4$ g of propane ($$\mathrm{C_3H_8}$$, molar mass 44.11$$\ \mathrm{g/mol}$$) is completely combusted, given $$\Delta H_{rxn} = -2044\ \mathrm{kJ}$$ per mole?

$$1.18 \times 10^4$\ \mathrm{g} \times \frac{1\ \mathrm{mol}}{44.11\ \mathrm{g}} \times (-2044\ \mathrm{kJ}) = -5.47 \times 10^5$\ \mathrm{kJ}$$

Q: Which of the following is the correct formula for calculating percent yield?

$$\text{Percent yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$$

Question 1

Which of the following best describes the greenhouse effect as it relates to global warming?

Accepted Answer

Greenhouse gases allow sunlight to enter the atmosphere but prevent heat from escaping, causing Earth's temperature to rise.

Question 2

Given the balanced equation for the combustion of octane: $$2\ \mathrm{C_8H_{18}}(l) + 25\ \mathrm{O_2$}(g) \rightarrow 16\ \mathrm{CO_2$}(g) + 18\ \mathrm{H_2O}(g)$$, how many moles of $$\mathrm{CO_2$}$$ are produced from 5 moles of $$\mathrm{C_8H_{18}}$$?

Accepted Answer

$$16\ \mathrm{mol\ CO_2$} 	imes \frac{5}{2} = 40\ \mathrm{mol\ CO_2$}$$

Question 3

A car burns $$1.00 	imes 10^4$ g of octane ($$\mathrm{C_8H_{18}}$$). Using the molar mass 114.3$$\ \mathrm{g/mol}$$ and the balanced equation 2$$\ \mathrm{C_8H_{18}} + 25\ \mathrm{$$O_2$$} ightarrow 16\ \mathrm{$$CO_2$$} + 18\ \mathrm{H_2O}$$, how many grams of $$\mathrm{$$CO_2$$}$$ are produced?

Accepted Answer

$$1.00 	imes 10^4$\ \mathrm{g\ C_8H_{18}} 	imes \frac{1\ \mathrm{mol}}{114.3\ \mathrm{g}} 	imes \frac{16\ \mathrm{mol\ CO_2$}}{2\ \mathrm{mol\ C_8H_{18}}} 	imes 44.01\ \mathrm{g\ CO_2$}/\mathrm{mol} = 3.08 	imes 10^4$\ \mathrm{g\ CO_2$}$$

Question 4

Which of the following statements about limiting reactants is correct?

Accepted Answer

The limiting reactant is the reactant that is completely consumed in a chemical reaction, thus limiting the amount of product formed.

Question 5

In the reaction $$2\ \mathrm{Na}(s) + \mathrm{Cl_2$}(g) \rightarrow 2\ \mathrm{NaCl}(s)$$, if you start with $$53.2\ \mathrm{g} Na $$and $$65.8\ \mathrm{g}$$ $$\mathrm{Cl_2$}$$, what is the theoretical yield of NaCl in grams?

Accepted Answer

$$108\ \mathrm{g}$$

Question 6

A reaction between $$11.5\ \mathrm{g}$$ $$\mathrm{Cu_2O}$$ and $$114.5\ \mathrm{g} C $$produces $$87.4\ \mathrm{g} Cu. If $$the theoretical yield is $$101.7\ \mathrm{g} Cu$$, what is the percent yield?

Accepted Answer

$$\frac{87.4}{101.7} 	imes 100\% = 85.9\%$$

Question 7

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction that releases heat to the surroundings, with $$\Delta H_{rxn} \u003c 0.$$

Question 8

The enthalpy of reaction for the combustion of methane is $$\Delta H_{rxn} = -802.3\ \mathrm{kJ}. $$What does the negative sign indicate?

Accepted Answer

The reaction releases 802.3 kJ of heat per mole of methane combusted.

Question 9

How much heat (in kJ) is released when $$1.18 	imes 10^4$ g of propane ($$\mathrm{C_3H_8}$$, molar mass 44.11$$\ \mathrm{g/mol}$$) is completely combusted, given $$\Delta H_{rxn} = -2044\ \mathrm{kJ}$$ per mole?

Accepted Answer

$$1.18 	imes 10^4$\ \mathrm{g} 	imes \frac{1\ \mathrm{mol}}{44.11\ \mathrm{g}} 	imes (-2044\ \mathrm{kJ}) = -5.47 	imes 10^5$\ \mathrm{kJ}$$

Question 10

A Bunsen burner flame is yellow and smoky. What adjustment should be made to achieve a hotter, blue flame?

Accepted Answer

Increase the amount of air entering the burner.

Question 11

Which of the following is the correct formula for calculating percent yield?

Accepted Answer

$$	ext{Percent yield} = \frac{	ext{actual yield}}{	ext{theoretical yield}} 	imes 100\%$$

Question 12

In the reaction $$N_2(g) + 3H_2(g$$) \rightarrow 2NH_3(g)$$, how many moles of NH_3$ can be produced from 6 moles of H_2$ and excess N_2$?

Accepted Answer

$$6\ \mathrm{mol\ H_2$} 	imes \frac{2\ \mathrm{mol\ NH_3$}}{3\ \mathrm{mol\ H_2$}} = 4\ \mathrm{mol\ NH_3$}$$

Question 13

Which of the following is true about the theoretical yield in a chemical reaction?

Accepted Answer

It is the maximum amount of product that can be formed from the limiting reactant.

Question 14

A reaction has an actual yield of $$86.4\ \mathrm{g}$$ and a theoretical yield of $$108\ \mathrm{g}. $$What is the percent yield?

Accepted Answer

$$\frac{86.4}{108} 	imes 100\% = 80.0\%$$

Question 15

Which of the following reactions is endothermic?

Accepted Answer

$$N_2(g$$) + $$O_2(g$$) \rightarrow 2NO(g)\ \Delta H_{rxn} = +182.6\ \mathrm{kJ}$$

Question 16

Which of the following best explains why the actual yield is usually less than the theoretical yield in a chemical reaction?

Accepted Answer

Some product is lost or does not form due to side reactions or incomplete reactions.

Ch 8; Quantities in Chemical Reactions: Stoichiometry, Limiting Reactants, and Enthalpy

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