Q: What is the percent composition by mass of oxygen in water ($H_2O$)? (Use atomic masses: H = 1.01, O = 16.00)

$\frac{16.00}{18.02} \times 100\% = 88.8\%$

Q: How many molecules of ibuprofen ($C_{13}H_{18}O_2$) are present in a 200 mg sample? (Molar mass = 206.29 g/mol)

$\frac{0.200\text{ g}}{206.29\text{ g/mol}} \times 6.022 \times 10^{23} = 5.84 \times 10^{20}$ molecules

Q: Which of the following is the correct definition of a mole?

A mole is the amount of substance that contains $6.022 \times 10^{23}$ particles.

Q: What is the percent yield if the theoretical yield of $NH_3$ is 53.9 g and the actual yield is 45.8 g?

$\frac{45.8}{53.9} \times 100\% = 85.0\%$

Q: What is the mass in grams of 12.4 moles of lead (Pb)? (Molar mass of Pb = 207.2 g/mol)

$12.4 \times 207.2 = 2,570.9$ g

Q: How many atoms are present in a 0.55 g sample of potassium (K)? (Molar mass = 39.10 g/mol, $N_A = 6.022 \times 10^{23}$)

$\frac{0.55}{39.10} \times 6.022 \times 10^{23} = 8.47 \times 10^{21}$ atoms

Question 1

Which of the following statements best describes the relationship between atomic mass units (amu) and molar mass for an element?

Accepted Answer

The atomic mass in amu is numerically equal to the molar mass in grams for one mole of atoms.

Question 2

A sample contains 8.56 g of carbon and 1.44 g of hydrogen. What is the empirical formula of the compound?

Accepted Answer

CH4

Question 3

What is the percent composition by mass of oxygen in water ($H_2O$)? (Use atomic masses: H = 1.01, O = 16.00)

Accepted Answer

$\frac{16.00}{18.02} 	imes 100\% = 88.8\%$

Question 4

How many molecules of ibuprofen ($C_{13}H_{18}O_2$) are present in a 200 mg sample? (Molar mass = 206.29 g/mol)

Accepted Answer

$\frac{0.200	ext{ g}}{206.29	ext{ g/mol}} 	imes 6.022 	imes 10^{23} = 5.84 	imes 10^{20}$ molecules

Question 5

Which of the following is the correct definition of a mole?

Accepted Answer

A mole is the amount of substance that contains $6.022 	imes 10^{23}$ particles.

Question 6

A chemist reacts 5.22 g of NO with 3.15 g of $O_2$ according to the equation $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$. What is the limiting reactant?

Accepted Answer

NO

Question 7

If 3.0 moles of $N_2$ react with 6.0 moles of $H_2$ to form $NH_3$ according to $N_2 + 3H_2 \rightarrow 2NH_3$, how many moles of $NH_3$ can be formed?

Accepted Answer

4.0 moles (from $H_2$), but $N_2$ limits to 4.0 moles

Question 8

What is the percent yield if the theoretical yield of $NH_3$ is 53.9 g and the actual yield is 45.8 g?

Accepted Answer

$\frac{45.8}{53.9} 	imes 100\% = 85.0\%$

Question 9

Which of the following best explains why chemists use excess of one reactant in industrial processes?

Accepted Answer

To ensure the more expensive reactant is completely consumed, maximizing product yield.

Question 10

A compound has an empirical formula of $CF_2$ and a molar mass of 200.0 g/mol. What is its molecular formula?

Accepted Answer

$C_4F_8$

Question 11

What is the mass in grams of 12.4 moles of lead (Pb)? (Molar mass of Pb = 207.2 g/mol)

Accepted Answer

$12.4 	imes 207.2 = 2,570.9$ g

Question 12

How many atoms are present in a 0.55 g sample of potassium (K)? (Molar mass = 39.10 g/mol, $N_A = 6.022 	imes 10^{23}$)

Accepted Answer

$\frac{0.55}{39.10} 	imes 6.022 	imes 10^{23} = 8.47 	imes 10^{21}$ atoms

Question 13

Which of the following is NOT a use of percent composition in real-world chemistry?

Accepted Answer

Measuring the temperature of a reaction

Question 14

A compound contains 62.04% carbon, 27.55% oxygen, and 10.41% hydrogen by mass. What is its empirical formula?

Accepted Answer

$C_3H_7O$

Question 15

Which conversion factor would you use to convert grams of a substance to moles?

Accepted Answer

Divide by the molar mass

Question 16

In the reaction $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$, how many grams of water are produced from 3.65 g of $H_2$ with excess $O_2$? (Molar mass $H_2 = 2.02$ g/mol, $H_2O = 18.02$ g/mol)

Accepted Answer

$3.65 \div 2.02 = 1.81$ mol $H_2$; $1.81$ mol $H_2$ produces $1.81$ mol $H_2O$; $1.81 	imes 18.02 = 32.6$ g $H_2O$

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\%$

Question 18

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

Accepted Answer

The reactant that determines the maximum amount of product formed.

The Mole Concept, Percent Composition, and Stoichiometry in General Chemistry

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