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

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