Q: Which of the following is true about percent yield?

Percent yield is calculated as $$(\text{actual yield}/\text{theoretical yield}) \times 100\%.$$

Question 1

Which of the following best describes the greenhouse effect?

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 octane, assuming excess oxygen?

Accepted Answer

$$40$$ moles

Question 3

A recipe for pancakes requires 2 eggs to make 5 pancakes. If you have 12 eggs and enough of all other ingredients, how many pancakes can you make?

Accepted Answer

30 pancakes

Question 4

In the reaction $$N_2(g) + 3H_2(g$$) \rightarrow 2NH_3(g)$$, how many moles of NH_3$ can be produced from 4 moles of N_2$ and excess H_2$?

Accepted Answer

8 moles

Question 5

Which of the following is the correct sequence for a mass-to-mass stoichiometry calculation?

Accepted Answer

mass A $$\rightarrow$$ moles A $$\rightarrow$$ moles B $$\rightarrow$$ mass B

Question 6

If you start with 53.2 g of Na and 65.8 g of $$Cl_2 in $$the reaction $$2Na(s) + Cl_2$(g) \rightarrow 2NaCl(s)$$, what is the limiting reactant?

Accepted Answer

$$Cl_2$$

Question 7

A reaction has a theoretical yield of 108 g NaCl, but only 86.4 g NaCl is actually produced. What is the percent yield?

Accepted Answer

$$80.0\%$$

Question 8

Which statement about exothermic and endothermic reactions is correct?

Accepted Answer

Exothermic reactions release heat and have a negative $$\Delta H_{rxn}.$$

Question 9

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.

Question 10

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

Accepted Answer

$$-5.47 	imes 10^5$ kJ

Question 11

A laboratory reaction produces 87.4 g of Cu from 11.5 g $$Cu_2O$$ and 114.5 g C. If the theoretical yield is 101.7 g Cu, what is the percent yield?

Accepted Answer

$$85.9\%$$

Question 12

Which of the following best defines the limiting reactant in a chemical reaction?

Accepted Answer

The reactant that is completely consumed and limits the amount of product formed.

Question 13

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

Accepted Answer

108 g

Question 14

Which of the following is a correct interpretation of the balanced equation $$N_2 + 3H_2$$ \rightarrow 2NH_3$$?

Accepted Answer

1 mole of $$N_2$$ reacts with 3 moles of $$H_2 to $$produce 2 moles of $$NH_3.$$

Question 15

Which of the following is true about percent yield?

Accepted Answer

Percent yield is calculated as $$(	ext{actual yield}/	ext{theoretical yield}) 	imes 100\%.$$

Question 16

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 17

Which of the following best describes the theoretical yield in a chemical reaction?

Accepted Answer

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

Question 18

If a reaction absorbs heat from the surroundings and $$\Delta H_{rxn} is $$positive, what type of reaction is it?

Accepted Answer

Endothermic

Question 19

Which of the following is NOT a greenhouse gas?

Accepted Answer

Oxygen ($$O_2)$$

Chapter 8: Quantities in Chemical Reactions – Stoichiometry, Limiting Reactants, and Enthalpy

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