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 Law of Conservation of Mass as it applies to chemical reactions?

Accepted Answer

The total mass of reactants must equal the total mass of products in a chemical reaction.

Question 2

Given the balanced equation for the combustion of methane: $$\mathrm{CH_4$(g) + 2\ O_2$(g) \rightarrow CO_2$(g) + 2\ H_2O(g)}$$, how many moles of water are produced from 3 moles of methane?

Accepted Answer

6 moles

Question 3

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

Accepted Answer

Mass A $$\rightarrow$$ Amount A (in moles) $$\rightarrow$$ Amount B (in moles) $$\rightarrow$$ Mass B

Question 4

A sample of octane ($$\mathrm{C_8H_{18}}) $$weighing $$4.0 	imes 10$$^{15}$$ g is burned completely. Using the balanced equation 2$$\ \mathrm{C_8H_{18}(l) + 25\ $$O_2(g$$) ightarrow 16\ $$CO_2(g) + 18$$\ H_2O(g)}$$, how many grams of $$\mathrm{$$CO_2$$}$$ are produced? (Molar masses: $$\mathrm{C_8H_{18}} = 114.22$$ g/mol, $$\mathrm{$$CO_2$$} = 44.01$$ g/mol)

Accepted Answer

$$1.2 	imes 10$$^{16}$$ g

Question 5

In a combustion analysis, all the carbon in an unknown compound is converted to $$\mathrm{CO_2$}$$ and all the hydrogen to $$\mathrm{H_2O}. If 1.00 g of $$the compound produces 2.20 g of $$\mathrm{CO_2$}$$ and 0.90 g of $$\mathrm{H_2O}$$, what is the empirical formula of the compound?

Accepted Answer

$$\mathrm{CH_2O}$$

Question 6

Which of the following statements about limiting reactants is correct?

Accepted Answer

The limiting reactant is the one that produces the smallest amount of product.

Question 7

A reaction mixture contains 42.5 g of Mg and 33.8 g of $$\mathrm{O_2$}. $$Using the reaction $$2\ \mathrm{Mg(s) + O_2$(g) \rightarrow 2\ MgO(s)}$$, what is the theoretical yield of $$\mathrm{MgO} in $$grams?

Accepted Answer

70.5 g

Question 8

If the actual yield of a reaction is 2.0 mol and the theoretical yield is 3.0 mol, what is the percent yield?

Accepted Answer

$$67\%$$

Question 9

Which of the following best represents a combustion reaction?

Accepted Answer

$$\mathrm{C_3H_8(g) + 5\ O_2$(g) \rightarrow 3\ CO_2$(g) + 4\ H_2O(g)}$$

Question 10

Which of the following is true about the reaction $$2\ \mathrm{H_2$(g) + O_2$(g) \rightarrow 2\ H_2O(g)}$$?

Accepted Answer

Two moles of hydrogen react with one mole of oxygen to produce two moles of water.

Question 11

A mixture of nitrogen and hydrogen gases reacts to form ammonia: $$\mathrm{N_2$(g) + 3\ H_2$(g) \rightarrow 2\ NH_3$(g)}. If $$you start with 1 mol $$\mathrm{N_2$}$$ and 2 mol $$\mathrm{H_2$}$$, which is the limiting reactant?

Accepted Answer

$$\mathrm{H_2$}$$

Question 12

Which of the following equations correctly represents the reaction of sodium with water?

Accepted Answer

$$2\ \mathrm{Na(s) + 2\ H_2O(l) \rightarrow 2\ NaOH(aq) + H_2$(g)}$$

Question 13

Which of the following is a correct statement about halogen reactions?

Accepted Answer

Halogens react with metals to form metal halides.

Question 14

In the reaction $$4\ \mathrm{$$FeCO_3$$(s) + O_2$(g) \rightarrow 2\ Fe_2O_3(s) + 4\ CO_2$(g)}$$, how many oxygen atoms are on the right-hand side of the equation?

Accepted Answer

10

Question 15

Suppose 5 mol $$\mathrm{NO_2$}$$ and 1 mol $$\mathrm{H_2O}$$ react according to $$3\ \mathrm{NO_2$(g) + H_2O(l) \rightarrow 2\ $$HNO_3$$(l) + NO(g)}. $$How many moles of the reactant in excess remain after the reaction is complete?

Accepted Answer

2 mol $$\mathrm{NO_2$}$$

Question 16

Which of the following best describes an excess reactant?

Accepted Answer

It is present in a greater amount than necessary to react with the limiting reactant.

Question 17

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

Accepted Answer

$$	ext{Percent yield} = \frac{	ext{Actual yield}}{	ext{Theoretical yield}} 	imes 100\%$$

CHEM 131 Chapter 4: Chemical Reactions and Chemical Quantities: Study Notes

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