Q: Which statement best describes the mole in chemistry?

A mole is a specific number of particles, $$6.022 \times 10$$^{23}$$, called Avogadro's number.

Q: How many atoms are in 2.5 moles of magnesium?

$$2.5 \times 6.022 \times 10$$^{23}$$ = 1.51 \times 10$$^{24}$$ atoms

Q: A sample contains $$1.2 \times 10$$^{24}$$ atoms of iron. How many moles of iron are present?

$$\frac{1.2 \times 10$$^{24}$$}{6.022 \times 10$$^{23}$$} = 1.99$$ mol

Q: What is the mass of 0.75 moles of water ($$H_2O$$)? (Molar mass of $$H_2O = 18.02 g/$$mol)

$$0.75 \times 18.02 = 13.52 g$$

Q: How many aluminum atoms are in a 10.0 g sample of aluminum? (Molar mass of Al = 26.98 g/mol)

$$\frac{10.0}{26.98} \times 6.022 \times 10$$^{23}$$ = 2.23 \times 10$$^{23}$$ atoms

Q: What is the mass of sodium in 15 g of sodium chloride ($$NaCl$$)? (Molar mass of $$NaCl = 58.44 g/$$mol, Na = 22.99 g/mol)

$$15 \times \frac{1}{58.44} \times \frac{22.99}{1} = 5.9 g$$

Q: A sample contains $$4.78 \times 10$$^{24}$$ molecules of NO_2$. What is the mass of this sample? (Molar mass of NO_2$ = 46.01 g/$$mol)

$$\frac{4.78 \times 10$$^{24}$$}{6.022 \times 10$$^{23}$$} \times 46.01 = 365 g$$

Q: Which conversion factor would you use to convert grams of aluminum to number of aluminum atoms?

$$\frac{1 \text{ mol Al}}{26.98 \text{ g Al}}$$ then $$\frac{6.022 \times 10$$^{23}$$ \text{ atoms}}{1 \text{ mol Al}}$$

Question 1

Which statement best describes the mole in chemistry?

Accepted Answer

A mole is a specific number of particles, $$6.022 	imes 10$$^{23}$$, called Avogadro's number.

Question 2

How many atoms are in 2.5 moles of magnesium?

Accepted Answer

$$2.5 	imes 6.022 	imes 10$$^{23}$$ = 1.51 	imes 10$$^{24}$$ atoms

Question 3

A sample contains $$1.2 	imes 10$$^{24}$$ atoms of iron. How many moles of iron are present?

Accepted Answer

$$\frac{1.2 	imes 10$$^{24}$$}{6.022 	imes 10$$^{23}$$} = 1.99$$ mol

Question 4

What is the mass of 0.75 moles of water ($$H_2O$$)? (Molar mass of $$H_2O = 18.02 g/$$mol)

Accepted Answer

$$0.75 	imes 18.02 = 13.52 g$$

Question 5

How many aluminum atoms are in a 10.0 g sample of aluminum? (Molar mass of Al = 26.98 g/mol)

Accepted Answer

$$\frac{10.0}{26.98} 	imes 6.022 	imes 10$$^{23}$$ = 2.23 	imes 10$$^{23}$$ atoms

Question 6

A diamond contains 0.58 g of pure carbon. How many moles of carbon are present? (Molar mass of C = 12.01 g/mol)

Accepted Answer

$$\frac{0.58}{12.01} = 0.048$$ mol

Question 7

What is the mass of sodium in 15 g of sodium chloride ($$NaCl$$)? (Molar mass of $$NaCl = 58.44 g/$$mol, Na = 22.99 g/mol)

Accepted Answer

$$15 	imes \frac{1}{58.44} 	imes \frac{22.99}{1} = 5.9 g$$

Question 8

How many moles of oxygen are in 2.5 moles of $$CO_2$$?

Accepted Answer

$$2.5 	imes 2 = 5.0$$ mol O

Question 9

A sample contains $$4.78 	imes 10$$^{24}$$ molecules of NO_2$. What is the mass of this sample? (Molar mass of NO_2$ = 46.01 g/$$mol)

Accepted Answer

$$\frac{4.78 	imes 10$$^{24}$$}{6.022 	imes 10$$^{23}$$} 	imes 46.01 = 365 g$$

Question 10

Which of the following best explains why the mass of 1 mole of sulfur is greater than the mass of 1 mole of carbon?

Accepted Answer

Sulfur atoms are heavier than carbon atoms, so 1 mole of sulfur has more mass.

Question 11

If you have 1.7 moles of $$CaCO_3$$, how many moles of oxygen atoms are present?

Accepted Answer

$$1.7 	imes 3 = 5.1$$ mol O

Question 12

Which conversion factor would you use to convert grams of aluminum to number of aluminum atoms?

Accepted Answer

$$\frac{1 	ext{ mol Al}}{26.98 	ext{ g Al}}$$ then $$\frac{6.022 	imes 10$$^{23}$$ 	ext{ atoms}}{1 	ext{ mol Al}}$$

Question 13

Which of the following is the correct empirical formula for a compound containing 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

Accepted Answer

CH$$_2O$$

Question 14

A hardware store sells nails by the pound. If a dozen nails weigh 0.150 lb, how many nails are in 2.60 lb?

Accepted Answer

$$2.60 \div 0.150 	imes 12 = 208$$ nails

Question 15

Which statement about molar mass is correct?

Accepted Answer

Molar mass is the mass of 1 mole of a substance, expressed in grams.

Question 16

How many moles of sodium are in 25 g of sodium chloride ($$NaCl$$)? (Molar mass of $$NaCl = 58.44 g/$$mol)

Accepted Answer

$$\frac{25}{58.44} = 0.428$$ mol NaCl, so 0.428 mol Na

Question 17

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

Accepted Answer

To convert grams of a substance directly to number of particles.

Question 18

A chemist wants to find the mass of chlorine in 50 g of $$CCl_4. ($$Molar mass of $$CCl_4 = 153.82$ g/mol, Cl = 35.45 g/mol)

Accepted Answer

$$50 \div 153.82 	imes 4 	imes 35.45 = 46.1 g$$

Chapter 6: Chemical Composition – Study Notes for Introductory Chemistry

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