Q: Which of the following best defines Avogadro’s number?

The number of particles in 1 mole of a substance, $6.022 \times 10^{23}$.

Q: A sample of nitrogen gas has a mass of 28.0 g. How many molecules are present in the sample? (Molar mass of $N_2 = 28.0$ g/mol)

$6.022 \times 10^{23}$ molecules

Q: Which of the following is the correct relationship to calculate the number of moles from mass and molar mass?

$\text{moles} = \dfrac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

Q: Which calculation would you use to determine the number of atoms in 2.00 moles of sodium?

$2.00 \times 6.022 \times 10^{23}$

Q: Which of the following is the correct formula to calculate percent composition by mass of an element in a compound?

$\dfrac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100\%$

Question 1

Which of the following best defines Avogadro’s number?

Accepted Answer

The number of particles in 1 mole of a substance, $6.022 	imes 10^{23}$.

Question 2

A sample contains $3.01 	imes 10^{23}$ molecules of $CO_2$. What is the mass of this sample? (Molar mass of $CO_2 = 44.0$ g/mol)

Accepted Answer

$22.0$ g

Question 3

Which of the following is the correct molar mass of $C_6H_{12}O_6$ (glucose)? (Atomic masses: C = 12.0, H = 1.0, O = 16.0)

Accepted Answer

$180.0$ g/mol

Question 4

At STP, what is the volume occupied by 2.00 moles of an ideal gas?

Accepted Answer

$44.8$ L

Question 5

Which of the following best describes the units and meaning of gas density at STP?

Accepted Answer

g/L; mass of gas per unit volume at standard temperature and pressure

Question 6

Calculate the percent composition by mass of oxygen in $H_2O$ (Atomic masses: H = 1.0, O = 16.0).

Accepted Answer

$88.9\%$

Question 7

A compound contains 40.0 g of carbon, 6.7 g of hydrogen, and 53.3 g of oxygen. What is its empirical formula? (Atomic masses: C = 12.0, H = 1.0, O = 16.0)

Accepted Answer

$CH_2O$

Question 8

A sample of nitrogen gas has a mass of 28.0 g. How many molecules are present in the sample? (Molar mass of $N_2 = 28.0$ g/mol)

Accepted Answer

$6.022 	imes 10^{23}$ molecules

Question 9

Which of the following is the correct relationship to calculate the number of moles from mass and molar mass?

Accepted Answer

$	ext{moles} = \dfrac{	ext{mass (g)}}{	ext{molar mass (g/mol)}}$

Question 10

A gas has a density of $1.25$ g/L at STP. What is its molar mass? (Molar volume at STP = $22.4$ L/mol)

Accepted Answer

$28.0$ g/mol

Question 11

A compound has an empirical formula of $CH_2$ and a molar mass of $56$ g/mol. What is its molecular formula? (Empirical formula mass = $14$ g/mol)

Accepted Answer

$C_4H_8$

Question 12

Which calculation would you use to determine the number of atoms in 2.00 moles of sodium?

Accepted Answer

$2.00 	imes 6.022 	imes 10^{23}$

Question 13

A 10.0 g sample of a compound contains 4.0 g of nitrogen and 6.0 g of oxygen. What is the percent composition of nitrogen?

Accepted Answer

$40.0\%$

Question 14

Which of the following statements about molar volume is correct?

Accepted Answer

At STP, 1 mole of any ideal gas occupies $22.4$ L.

Question 15

A sample of $O_2$ gas occupies $11.2$ L at STP. How many moles of $O_2$ are present?

Accepted Answer

$0.5$ mol

Question 16

A chemist analyzes a compound and finds it contains 24.3% carbon, 4.1% hydrogen, and 71.6% chlorine by mass. What is the empirical formula? (Atomic masses: C = 12.0, H = 1.0, Cl = 35.5)

Accepted Answer

$CH_2Cl$

Question 17

Which of the following best explains why the empirical formula is not always the same as the molecular formula?

Accepted Answer

The empirical formula shows the simplest whole-number ratio of atoms, while the molecular formula shows the actual number of atoms in a molecule.

Question 18

A 5.00 g sample of a gas occupies 2.00 L at STP. What is the molar mass of the gas? (Molar volume at STP = $22.4$ L/mol)

Accepted Answer

$56.0$ g/mol

Question 19

Which of the following is the correct formula to calculate percent composition by mass of an element in a compound?

Accepted Answer

$\dfrac{	ext{mass of element in 1 mol of compound}}{	ext{molar mass of compound}} 	imes 100\%$

Question 20

A compound has a molar mass of $58.0$ g/mol and an empirical formula of $C_2H_5$. What is its molecular formula? (Empirical formula mass = $29.0$ g/mol)

Accepted Answer

$C_4H_{10}$

Chapter 8: The Mole Concept – Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards