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

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

Q: Which equation correctly relates mass, molar mass, and moles?

$$\text{moles} = \dfrac{\text{mass}}{\text{molar mass}}$$

Q: Which formula is used to calculate the percent composition of an element in a compound?

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

Q: Which of the following best describes the relationship between molar mass and density for a gas at STP?

$$\text{Density} = \dfrac{\text{Molar Mass}}{22.4}$$

Question 1

Which of the following best describes 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 water. How many moles of water are present?

Accepted Answer

$$0.50$$ moles

Question 3

What is the molar mass of $$\mathrm{C_2H_6O} ($$ethanol)?

Accepted Answer

$$46.07 g/$$mol

Question 4

At STP, what is the volume occupied by $$2$$ moles of an ideal gas?

Accepted Answer

$$44.8 L$$

Question 5

Which equation correctly relates mass, molar mass, and moles?

Accepted Answer

$$	ext{moles} = \dfrac{	ext{mass}}{	ext{molar mass}}$$

Question 6

A $$10.0 g $$sample of $$\mathrm{CO_2$}$$ contains how many molecules?

Accepted Answer

$$1.37 	imes 10$$^{24}$$

Question 7

What is the percent composition by mass of oxygen in $$\mathrm{H_2O}$$?

Accepted Answer

$$88.8\%$$

Question 8

A gas has a density of $$1.25 g/L at $$STP. What is its molar mass?

Accepted Answer

$$28.0 g/$$mol

Question 9

Which of the following statements about molar volume at STP is correct?

Accepted Answer

One mole of any ideal gas occupies $$22.4 L at $$STP.

Question 10

A compound contains $$40.0\%$$ carbon, $$6.7\%$$ hydrogen, and $$53.3\%$$ oxygen by mass. What is the empirical formula?

Accepted Answer

$$\mathrm{CH_2O}$$

Question 11

If you have $$0.25$$ moles of $$\mathrm{NaCl}$$, how many formula units do you have?

Accepted Answer

$$1.51 	imes 10$$^{23}$$

Question 12

Which of the following best explains why gases have lower densities than solids and liquids?

Accepted Answer

Gas particles are much farther apart, resulting in less mass per unit volume.

Question 13

A $$5.00 g $$sample of $$\mathrm{N_2$}$$ gas occupies what volume at STP?

Accepted Answer

$$4.00 L$$

Question 14

Which formula is used to calculate the percent composition of an element in a compound?

Accepted Answer

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

Question 15

A $$2.00 L $$sample of oxygen gas at STP contains how many molecules?

Accepted Answer

$$5.38 	imes 10$$^{22}$$

Question 16

Which of the following is the correct unit for gas density?

Accepted Answer

g/L

Question 17

A $$20.0 g $$sample of $$\mathrm{O_2$}$$ gas contains how many moles?

Accepted Answer

$$0.625$$ moles

Question 18

Which of the following is NOT a correct use of Avogadro’s number?

Accepted Answer

To convert grams to liters at STP.

Question 19

A $$12.0 g $$sample of methane ($$\mathrm{CH_4$}) $$contains how many hydrogen atoms?

Accepted Answer

$$1.80 	imes 10$$^{24}$$

Question 20

Which of the following best describes the relationship between molar mass and density for a gas at STP?

Accepted Answer

$$	ext{Density} = \dfrac{	ext{Molar Mass}}{22.4}$$

Quantities in Chemical Reactions: Avogadro’s Number, Moles, Molar Mass, Molar Volume, Gas Density, and Percent Composition

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