Q: A reaction has a theoretical yield of 10.0 g, but only 7.5 g of product is actually obtained. What is the percent yield?

$\displaystyle \frac{7.5}{10.0} \times 100 = 75\%$

Question 1

Which of the following best describes a chemical change?

Accepted Answer

Iron rusting when exposed to oxygen.

Question 2

Which of the following is a balanced chemical equation for the combustion of methane ($\mathrm{CH_4}$)?

Accepted Answer

$\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$

Question 3

A student burns 16 g of methane ($\mathrm{CH_4}$) in excess oxygen. How many grams of carbon dioxide ($\mathrm{CO_2}$) are produced? (Molar masses: $\mathrm{CH_4} = 16\ \mathrm{g/mol}$, $\mathrm{CO_2} = 44\ \mathrm{g/mol}$)

Accepted Answer

44 g

Question 4

Which of the following statements about limiting reactants is correct?

Accepted Answer

The limiting reactant is the one that is completely consumed first in a reaction.

Question 5

Which of the following is a physical change?

Accepted Answer

Dissolving sugar in water.

Question 6

In the reaction $2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$, how many moles of water are produced from 5 moles of $\mathrm{O_2}$ and excess $\mathrm{H_2}$?

Accepted Answer

10 moles

Question 7

Which of the following best explains why greenhouse gases contribute to global warming?

Accepted Answer

They absorb and re-emit infrared radiation, trapping heat in the atmosphere.

Question 8

A reaction has a theoretical yield of 10.0 g, but only 7.5 g of product is actually obtained. What is the percent yield?

Accepted Answer

$\displaystyle \frac{7.5}{10.0} 	imes 100 = 75\%$

Question 9

Which of the following is the correct mole-to-mole conversion factor for the reaction $2\mathrm{C_8H_{18}} + 25\mathrm{O_2} \rightarrow 16\mathrm{CO_2} + 18\mathrm{H_2O}$ to find moles of $\mathrm{CO_2}$ produced per mole of $\mathrm{C_8H_{18}}$ burned?

Accepted Answer

$\displaystyle \frac{16\ \mathrm{mol}\ \mathrm{CO_2}}{2\ \mathrm{mol}\ \mathrm{C_8H_{18}}}$

Question 10

Which of the following is a correct description of a combustion reaction?

Accepted Answer

A reaction in which a substance reacts with oxygen to produce heat and light.

Question 11

If 10.0 g of $\mathrm{Mg(OH)_2}$ (molar mass = 58.3 g/mol) reacts with excess HCl, how many moles of $\mathrm{Mg(OH)_2}$ are present?

Accepted Answer

$0.172\ \mathrm{mol}$

Question 12

Which of the following is NOT a property of a physical change?

Accepted Answer

Change in color due to a new substance forming.

Question 13

A pizza recipe requires 3 cups of flour for every 2 cups of sauce. If you have 12 cups of flour and 10 cups of sauce, what is the limiting ingredient?

Accepted Answer

Sauce

Question 14

Which of the following best describes the role of a balanced chemical equation in stoichiometry?

Accepted Answer

It provides the mole ratios needed to relate reactants and products.

Question 15

In the reaction $2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{NaOH} + \mathrm{H_2}$, what is the theoretical yield of $\mathrm{H_2}$ (in moles) if 4 moles of $\mathrm{Na}$ react with excess $\mathrm{H_2O}$?

Accepted Answer

2 moles

Question 16

Which of the following is a correct statement about alkali metal reactions with water?

Accepted Answer

They produce hydrogen gas and a metal hydroxide.

Question 17

A sample of $\mathrm{C_8H_{18}}$ (octane) is combusted, producing 35.2 g of $\mathrm{CO_2}$. How many moles of $\mathrm{CO_2}$ is this? (Molar mass $\mathrm{CO_2} = 44.0\ \mathrm{g/mol}$)

Accepted Answer

0.80 mol

Question 18

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

Accepted Answer

$\displaystyle \frac{	ext{actual yield}}{	ext{theoretical yield}} 	imes 100$

Question 19

Which of the following best describes a halogen reaction with a metal?

Accepted Answer

A halogen gains electrons to form a negative ion.

Chapter 7: Chemical Reactions - Stoichiometry

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