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

$$-301 \ \mathrm{kJ}$$

Thermochemical Stoichiometry and Thermochemical Equations: Study Notes

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