Q: Which of the following is the correct formula to convert grams of a substance to moles?

$\text{moles} = \frac{\text{grams}}{\text{molar mass}}$

Question 1

Which of the following best describes the purpose of a thermochemical equation?

Accepted Answer

To indicate the enthalpy change associated with a chemical reaction.

Question 2

Given the reaction $2 \mathrm{Mg} (s) + \mathrm{O}_2 (g) \rightarrow 2 \mathrm{MgO} (s)$ with $\Delta H = -1204 \ \mathrm{kJ}$, how much heat is released when 24.3 g of Mg reacts completely?

Accepted Answer

$-480 \ \mathrm{kJ}$

Question 3

In the reaction $4 \mathrm{CH}_3\mathrm{NO}_2 (l) + 7 \mathrm{O}_2 (g) \rightarrow 4 \mathrm{CO}_2 (g) + 6 \mathrm{H}_2\mathrm{O} (g) + 4 \mathrm{NO}_2 (g)$, $\Delta H = -2441.6 \ \mathrm{kJ}$, what is the first step to determine the heat released by burning 125.0 g of nitromethane?

Accepted Answer

Convert grams of nitromethane to moles.

Question 4

A student wants to know how much heat is released when 250.0 g of CO reacts with excess H$_2$ according to $\mathrm{CO} (g) + 2 \mathrm{H}_2 (g) \rightarrow \mathrm{CH}_3\mathrm{OH} (l)$, $\Delta H = -128.1 \ \mathrm{kJ}$. What is the correct sequence of steps?

Accepted Answer

Convert grams CO to moles CO, use mole ratio, multiply by $\Delta H$.

Question 5

If 61.044 g/mol is the molar mass of nitromethane, how many moles are present in 125.0 g?

Accepted Answer

$2.05$ moles

Question 6

For the reaction $2 \mathrm{C}_6\mathrm{H}_6 (l) + 15 \mathrm{O}_2 (g) ightarrow 12 \mathrm{CO}_2 (g) + 6 \mathrm{H}_2\mathrm{O} (g)$, $\Delta H = -6278 \ \mathrm{kJ}$, what volume of benzene (density = $0.880 \ \mathrm{g/mL}$, molar mass = $78.11 \ \mathrm{g/mol}$) is needed to release $5.19 	imes 10^3 \ \mathrm{kJ}$ of heat?

Accepted Answer

$81.7 \ \mathrm{mL}$

Question 7

Why is it important to use the coefficients from the balanced equation when performing mole-to-mole conversions in stoichiometry?

Accepted Answer

They show the ratio in which substances react and are produced.

Question 8

A reaction releases $-1204 \ \mathrm{kJ}$ for every $2$ moles of Mg reacted. What is the enthalpy change when $1$ mole of Mg reacts?

Accepted Answer

$-602 \ \mathrm{kJ}$

Question 9

In a real-world scenario, why might a chemist need to convert grams of a reactant to moles before using a thermochemical equation?

Accepted Answer

Because thermochemical equations are based on mole ratios, not mass.

Question 10

If $2.32 	imes 10^3$ g of H$_2$ is used in the reaction $\mathrm{CO} (g) + 2 \mathrm{H}_2 (g) ightarrow \mathrm{CH}_3\mathrm{OH} (l)$, how many moles of H$_2$ is this?

Accepted Answer

$1.15 	imes 10^3$ moles

Question 11

Which of the following is the correct formula to convert grams of a substance to moles?

Accepted Answer

$	ext{moles} = \frac{	ext{grams}}{	ext{molar mass}}$

Question 12

A student calculates the heat released from burning 125.0 g of nitromethane and gets $-5000 \ \mathrm{kJ}$. What is the most likely error?

Accepted Answer

They forgot to convert grams to moles before using $\Delta H$.

Question 13

In the reaction $2 \mathrm{C}_6\mathrm{H}_6 (l) + 15 \mathrm{O}_2 (g) \rightarrow 12 \mathrm{CO}_2 (g) + 6 \mathrm{H}_2\mathrm{O} (g)$, what is the mole ratio of benzene to carbon dioxide?

Accepted Answer

$2:12$

Question 14

Which step in the stoichiometric process involves using the coefficients from the balanced equation?

Accepted Answer

Mole to mole comparison.

Question 15

If a reaction has $\Delta H = -6278 \ \mathrm{kJ}$ for $2$ moles of benzene, how much heat is released per mole?

Accepted Answer

$-3139 \ \mathrm{kJ}$

Question 16

A chemist needs to produce $2.51 \ \mathrm{g}$ of MgO. How many moles of MgO is this? (Molar mass = $40.3 \ \mathrm{g/mol}$)

Accepted Answer

$0.062$ moles

Question 17

Which of the following is a real-world application of thermochemical equations?

Accepted Answer

Calculating the energy released in fuel combustion for vehicles.

Question 18

If $2 \mathrm{Mg} (s) + \mathrm{O}_2 (g) \rightarrow 2 \mathrm{MgO} (s)$ releases $-1204 \ \mathrm{kJ}$, how much heat is released when $0.5$ moles of Mg reacts?

Accepted Answer

$-301 \ \mathrm{kJ}$

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