Q: What is the formula mass (in g/mol) of $\mathrm{Ca(NO_3)_2}$? (Atomic masses: Ca = 40.08, N = 14.01, O = 16.00)

$40.08 + 2 \times (14.01 + 3 \times 16.00) = 164.10\ \mathrm{g/mol}$

Q: A sample contains $2.7\ \mathrm{mol}$ of $\mathrm{CO_2}$. What is the mass of this sample in grams? (Molar mass of $\mathrm{CO_2} = 44.01\ \mathrm{g/mol}$)

$2.7 \times 44.01 = 118.83\ \mathrm{g}$

Q: What is the mass percent of oxygen in acetic acid ($\mathrm{C_2H_4O_2}$)? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

$\frac{2 \times 16.00}{60.05} \times 100 = 53.28\%$

Q: A tablet contains $200.0\ \mathrm{mg}$ of ibuprofen ($\mathrm{C_{13}H_{18}O_2}$). How many molecules of ibuprofen are present? (Molar mass = $206.285\ \mathrm{g/mol}$, $N_A = 6.022 \times 10^{23}\ \mathrm{mol}^{-1}$)

$\frac{0.2000}{206.285} \times 6.022 \times 10^{23} = 5.83 \times 10^{20}$ molecules

Q: Calculate the mass percent composition of sodium in sodium oxide ($\mathrm{Na_2O}$). (Atomic masses: Na = 22.99, O = 16.00)

$\frac{2 \times 22.99}{2 \times 22.99 + 16.00} \times 100 = 79.12\%$

Q: If a person consumes $22\ \mathrm{g}$ of sodium chloride per day, how many grams of sodium does this person consume? (Sodium chloride is 39% sodium by mass)

$22 \times 0.39 = 8.6\ \mathrm{g}$

Q: A sample of water contains $3.55 \times 10^{24}$ molecules. What is the mass of this sample in grams? (Molar mass of $\mathrm{H_2O} = 18.015\ \mathrm{g/mol}$, $N_A = 6.022 \times 10^{23}\ \mathrm{mol}^{-1}$)

$\frac{3.55 \times 10^{24}}{6.022 \times 10^{23}} \times 18.015 = 106.1\ \mathrm{g}$

Question 1

Which of the following correctly gives the number of each type of atom in the compound $\mathrm{Mg_3(PO_4)_2}$?

Accepted Answer

Mg: 3, P: 2, O: 8

Question 2

What is the formula mass (in g/mol) of $\mathrm{Ca(NO_3)_2}$? (Atomic masses: Ca = 40.08, N = 14.01, O = 16.00)

Accepted Answer

$40.08 + 2 	imes (14.01 + 3 	imes 16.00) = 164.10\ \mathrm{g/mol}$

Question 3

A sample contains $2.7\ \mathrm{mol}$ of $\mathrm{CO_2}$. What is the mass of this sample in grams? (Molar mass of $\mathrm{CO_2} = 44.01\ \mathrm{g/mol}$)

Accepted Answer

$2.7 	imes 44.01 = 118.83\ \mathrm{g}$

Question 4

What is the mass percent of oxygen in acetic acid ($\mathrm{C_2H_4O_2}$)? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

Accepted Answer

$\frac{2 	imes 16.00}{60.05} 	imes 100 = 53.28\%$

Question 5

A tablet contains $200.0\ \mathrm{mg}$ of ibuprofen ($\mathrm{C_{13}H_{18}O_2}$). How many molecules of ibuprofen are present? (Molar mass = $206.285\ \mathrm{g/mol}$, $N_A = 6.022 	imes 10^{23}\ \mathrm{mol}^{-1}$)

Accepted Answer

$\frac{0.2000}{206.285} 	imes 6.022 	imes 10^{23} = 5.83 	imes 10^{20}$ molecules

Question 6

Calculate the mass percent composition of sodium in sodium oxide ($\mathrm{Na_2O}$). (Atomic masses: Na = 22.99, O = 16.00)

Accepted Answer

$\frac{2 	imes 22.99}{2 	imes 22.99 + 16.00} 	imes 100 = 79.12\%$

Question 7

If a person consumes $22\ \mathrm{g}$ of sodium chloride per day, how many grams of sodium does this person consume? (Sodium chloride is 39% sodium by mass)

Accepted Answer

$22 	imes 0.39 = 8.6\ \mathrm{g}$

Question 8

A sample of water contains $3.55 	imes 10^{24}$ molecules. What is the mass of this sample in grams? (Molar mass of $\mathrm{H_2O} = 18.015\ \mathrm{g/mol}$, $N_A = 6.022 	imes 10^{23}\ \mathrm{mol}^{-1}$)

Accepted Answer

$\frac{3.55 	imes 10^{24}}{6.022 	imes 10^{23}} 	imes 18.015 = 106.1\ \mathrm{g}$

Question 9

Which of the following steps is NOT necessary when calculating the formula mass of a compound?

Accepted Answer

Dividing the total mass by Avogadro's number

Question 10

A $128\ \mathrm{kg}$ sample of carbon tetrabromide ($\mathrm{CBr_4}$) is used in an industrial process. How many moles of $\mathrm{CBr_4}$ are present? (Molar mass = $331.63\ \mathrm{g/mol}$)

Accepted Answer

$\frac{128,000}{331.63} = 386.0\ \mathrm{mol}$

Question 11

Which of the following best describes the process for finding the mass percent of an element in a compound?

Accepted Answer

Divide the total mass of the element in one mole of the compound by the molar mass of the compound, then multiply by 100

Question 12

A $112.7\ \mathrm{mmol}$ sample of sulfur hexafluoride ($\mathrm{SF_6}$) is used in a laboratory. What is the mass of this sample in grams? (Molar mass = $146.06\ \mathrm{g/mol}$)

Accepted Answer

$0.1127 	imes 146.06 = 16.46\ \mathrm{g}$

Question 13

Which of the following is the correct formula mass for $\mathrm{H_2SO_4}$? (Atomic masses: H = 1.01, S = 32.07, O = 16.00)

Accepted Answer

$2 	imes 1.01 + 32.07 + 4 	imes 16.00 = 98.09\ \mathrm{g/mol}$

Question 14

A $38.6\ \mathrm{g}$ sample of nitrogen dioxide ($\mathrm{NO_2}$) contains how many moles? (Molar mass = $46.01\ \mathrm{g/mol}$)

Accepted Answer

$\frac{38.6}{46.01} = 0.839\ \mathrm{mol}$

Question 15

Which of the following best explains why the mass percent of sodium in sodium chloride is less than 50%?

Accepted Answer

Chlorine has a much higher atomic mass than sodium, so it contributes more to the total mass of the compound

Question 16

If you have $41.009\ \mathrm{g}$ of $\mathrm{CO_2}$, how many moles of $\mathrm{CO_2}$ do you have? (Molar mass = $44.01\ \mathrm{g/mol}$)

Accepted Answer

$\frac{41.009}{44.01} = 0.932\ \mathrm{mol}$

Formula Mass and Mass Percent Composition

Study Guide - Practice Questions

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