Q: A mining company extracts 500 kg of iron ore that is 60% pure Fe2O3 by mass. What is the mass of iron (Fe) present in the ore? (Molar mass: Fe2O3 = 159.7 g/mol, Fe = 55.85 g/mol)

$$\text{Mass of Fe} = 500\,\text{kg} \times 0.60 \times \frac{2 \times 55.85\,\text{g/mol}}{159.7\,\text{g/mol}} = 210\,\text{kg}$$

Q: A can of Freon-12 (CCl2F2) contains 250 g of the compound. How many moles of chlorine atoms are present in the can? (Molar mass: CCl2F2 = 120.91 g/mol)

$$\text{Moles of Cl} = 250\,\text{g} \times \frac{1\,\text{mol}}{120.91\,\text{g}} \times 2 = 4.14\,\text{mol}$$

Q: A jeweler has a silver ring containing $$1.1 \times 10$$^{22}$$ silver atoms. How many moles of silver are in the ring?

$$\text{Moles of Ag} = \frac{1.1 \times 10$$^{22}$$}{6.022 \times 10$$^{23}$$} = 1.8 \times 10$$^{-2}$$\,\text{mol}$$

Q: How many atoms are present in a 0.58 g diamond? (Assume pure carbon, molar mass = 12.01 g/mol)

$$0.58\,\text{g} \times \frac{1\,\text{mol}}{12.01\,\text{g}} \times 6.022 \times 10$$^{23}$$\,\text{atoms/mol} = 2.9 \times 10$$^{22}$$\,\text{atoms}$$

Q: A typical aluminum can has a mass of 16.2 g. How many aluminum atoms does it contain? (Molar mass of Al = 26.98 g/mol)

$$16.2\,\text{g} \times \frac{1\,\text{mol}}{26.98\,\text{g}} \times 6.022 \times 10$$^{23}$$\,\text{atoms/mol} = 3.62 \times 10$$^{23}$$\,\text{atoms}$$

Q: How many grams of water are present in 1.75 mol of H2O? (Molar mass of H2O = 18.02 g/mol)

$$1.75\,\text{mol} \times 18.02\,\text{g/mol} = 31.0\,\text{g}$$

Q: A sample contains $$4.78 \times 10$$^{24}$$ molecules of NO2. What is the mass of the sample? (Molar mass of NO2 = 46.01 g/mol)

$$4.78 \times 10$$^{24}$$ \times \frac{1\,\text{mol}}{6.022 \times 10$$^{23}$$} \times 46.01\,\text{g/mol} = 365\,\text{g}$$

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

$$6.022 \times 10$$^{23}$$, the number of particles in one mole of a substance

Q: If you have 32.07 g of sulfur, how many moles of sulfur do you have? (Molar mass of S = 32.07 g/mol)

$$\frac{32.07\,\text{g}}{32.07\,\text{g/mol}} = 1.00\,\text{mol}$$

Q: A chemist needs 0.250 mol of carbon for an experiment. How many grams of carbon should be weighed out? (Molar mass of C = 12.01 g/mol)

$$0.250\,\text{mol} \times 12.01\,\text{g/mol} = 3.00\,\text{g}$$

Question 1

A mining company extracts 500 kg of iron ore that is 60% pure Fe2O3 by mass. What is the mass of iron (Fe) present in the ore? (Molar mass: Fe2O3 = 159.7 g/mol, Fe = 55.85 g/mol)

Accepted Answer

$$	ext{Mass of Fe} = 500\,	ext{kg} 	imes 0.60 	imes \frac{2 	imes 55.85\,	ext{g/mol}}{159.7\,	ext{g/mol}} = 210\,	ext{kg}$$

Question 2

A can of Freon-12 (CCl2F2) contains 250 g of the compound. How many moles of chlorine atoms are present in the can? (Molar mass: CCl2F2 = 120.91 g/mol)

Accepted Answer

$$	ext{Moles of Cl} = 250\,	ext{g} 	imes \frac{1\,	ext{mol}}{120.91\,	ext{g}} 	imes 2 = 4.14\,	ext{mol}$$

Question 3

A jeweler has a silver ring containing $$1.1 	imes 10$$^{22}$$ silver atoms. How many moles of silver are in the ring?

Accepted Answer

$$	ext{Moles of Ag} = \frac{1.1 	imes 10$$^{22}$$}{6.022 	imes 10$$^{23}$$} = 1.8 	imes 10$$^{-2}$$\,	ext{mol}$$

Question 4

How many atoms are present in a 0.58 g diamond? (Assume pure carbon, molar mass = 12.01 g/mol)

Accepted Answer

$$0.58\,	ext{g} 	imes \frac{1\,	ext{mol}}{12.01\,	ext{g}} 	imes 6.022 	imes 10$$^{23}$$\,	ext{atoms/mol} = 2.9 	imes 10$$^{22}$$\,	ext{atoms}$$

Question 5

A typical aluminum can has a mass of 16.2 g. How many aluminum atoms does it contain? (Molar mass of Al = 26.98 g/mol)

Accepted Answer

$$16.2\,	ext{g} 	imes \frac{1\,	ext{mol}}{26.98\,	ext{g}} 	imes 6.022 	imes 10$$^{23}$$\,	ext{atoms/mol} = 3.62 	imes 10$$^{23}$$\,	ext{atoms}$$

Question 6

How many grams of water are present in 1.75 mol of H2O? (Molar mass of H2O = 18.02 g/mol)

Accepted Answer

$$1.75\,	ext{mol} 	imes 18.02\,	ext{g/mol} = 31.0\,	ext{g}$$

Question 7

A sample contains $$4.78 	imes 10$$^{24}$$ molecules of NO2. What is the mass of the sample? (Molar mass of NO2 = 46.01 g/mol)

Accepted Answer

$$4.78 	imes 10$$^{24}$$ 	imes \frac{1\,	ext{mol}}{6.022 	imes 10$$^{23}$$} 	imes 46.01\,	ext{g/mol} = 365\,	ext{g}$$

Question 8

Which of the following best describes Avogadro’s number?

Accepted Answer

$$6.022 	imes 10$$^{23}$$, the number of particles in one mole of a substance

Question 9

If you have 32.07 g of sulfur, how many moles of sulfur do you have? (Molar mass of S = 32.07 g/mol)

Accepted Answer

$$\frac{32.07\,	ext{g}}{32.07\,	ext{g/mol}} = 1.00\,	ext{mol}$$

Question 10

A chemist needs 0.250 mol of carbon for an experiment. How many grams of carbon should be weighed out? (Molar mass of C = 12.01 g/mol)

Accepted Answer

$$0.250\,	ext{mol} 	imes 12.01\,	ext{g/mol} = 3.00\,	ext{g}$$

Question 11

Which of the following statements about the mole is correct?

Accepted Answer

One mole of any element contains $$6.022 	imes 10$$^{23}$$ atoms of that element.

Question 12

A sample contains 0.75 mol of glucose (C6H12O6). How many molecules of glucose are present?

Accepted Answer

$$0.75\,	ext{mol} 	imes 6.022 	imes 10$$^{23}$$\,	ext{molecules/mol} = 4.52 	imes 10$$^{23}$$\,	ext{molecules}$$

Question 13

A hardware store sells 2.60 lb of medium-sized nails, and a dozen of these nails weigh 0.150 lb. How many nails did the customer buy?

Accepted Answer

$$2.60\,	ext{lb} 	imes \frac{1\,	ext{doz}}{0.150\,	ext{lb}} 	imes 12 = 208\,	ext{nails}$$

Question 14

Which of the following best explains why the mass of one mole of different elements is not the same?

Accepted Answer

Different elements have different atomic masses, so one mole of each element has a different mass.

Question 15

Calculate the mass percent of oxygen in water (H2O). (Atomic masses: H = 1.01 g/mol, O = 16.00 g/mol)

Accepted Answer

$$\frac{16.00}{2 	imes 1.01 + 16.00} 	imes 100\% = 88.8\%$$

Question 16

A chemist determines that a compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the empirical formula of the compound?

Accepted Answer

CH2O

Chapter 6: Chemical Composition – Moles, Mass, and Counting Atoms

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