Q: What is Avogadro's number, and what does it represent?

$$6.022 \times 10$$^{23}$$, representing the number of atoms or molecules in one mole of a substance.

Q: How many atoms are present in 2.00 moles of helium?

$$2.00 \times 6.022 \times 10$$^{23}$$ = 1.2044 \times 10$$^{24}$$ atoms

Q: How many aluminum atoms are present in a 54.0 g aluminum can? (Molar mass of Al = 26.98 g/mol)

$$\frac{54.0}{26.98} \times 6.022 \times 10$$^{23}$$ = 2.00 \times 6.022 \times 10$$^{23}$$ = 1.20 \times 10$$^{24}$$ atoms

Q: A sample contains $$3.01 \times 10$$^{23}$$ silver atoms. How many moles of silver does this represent?

$$\frac{3.01 \times 10$$^{23}$$}{6.022 \times 10$$^{23}$$} = 0.500$$ mol

Q: Which of the following is the correct conversion factor to go from grams of an element to number of atoms?

$$\frac{1 \text{ mol}}{\text{molar mass in g}} \times \frac{6.022 \times 10$$^{23}$$ \text{ atoms}}{1 \text{ mol}}$$

Question 1

Which statement best explains why chemists use the mass of atoms to count them instead of counting each atom individually?

Accepted Answer

Atoms are too small and numerous to count individually, so mass provides a practical way to quantify them.

Question 2

What is Avogadro's number, and what does it represent?

Accepted Answer

$$6.022 	imes 10$$^{23}$$, representing the number of atoms or molecules in one mole of a substance.

Question 3

If you have 0.58 g of pure carbon, how many moles of carbon do you have? (Atomic mass of carbon = 12.01 g/mol)

Accepted Answer

$$0.58 \div 12.01 = 0.048$$ mol

Question 4

How many atoms are present in 2.00 moles of helium?

Accepted Answer

$$2.00 	imes 6.022 	imes 10$$^{23}$$ = 1.2044 	imes 10$$^{24}$$ atoms

Question 5

A can contains 54.0 g of aluminum. How many moles of aluminum are in the can? (Molar mass of Al = 26.98 g/mol)

Accepted Answer

$$54.0 \div 26.98 = 2.00$$ mol

Question 6

How many aluminum atoms are present in a 54.0 g aluminum can? (Molar mass of Al = 26.98 g/mol)

Accepted Answer

$$\frac{54.0}{26.98} 	imes 6.022 	imes 10$$^{23}$$ = 2.00 	imes 6.022 	imes 10$$^{23}$$ = 1.20 	imes 10$$^{24}$$ atoms

Question 7

Which of the following best describes the relationship between moles, mass, and number of atoms?

Accepted Answer

Moles connect the mass of a substance to the number of atoms using the molar mass and Avogadro's number.

Question 8

A nutrition label recommends no more than 2.3 g of sodium per day. If sodium chloride (NaCl) is 39.3% sodium by mass, what is the maximum mass of NaCl you can consume to stay within the sodium recommendation?

Accepted Answer

$$2.3 \div 0.393 = 5.85 g $$NaCl

Question 9

What is the molar mass of sodium chloride (NaCl)? (Atomic masses: Na = 22.99 g/mol, Cl = 35.45 g/mol)

Accepted Answer

$$22.99 + 35.45 = 58.44 g/$$mol

Question 10

If you have 1.5 moles of copper atoms, what is the mass of copper you have? (Molar mass of Cu = 63.55 g/mol)

Accepted Answer

$$1.5 	imes 63.55 = 95.33 g$$

Question 11

A sample contains $$3.01 	imes 10$$^{23}$$ silver atoms. How many moles of silver does this represent?

Accepted Answer

$$\frac{3.01 	imes 10$$^{23}$$}{6.022 	imes 10$$^{23}$$} = 0.500$$ mol

Question 12

Which of the following is the correct conversion factor to go from grams of an element to number of atoms?

Accepted Answer

$$\frac{1 	ext{ mol}}{	ext{molar mass in g}} 	imes \frac{6.022 	imes 10$$^{23}$$ 	ext{ atoms}}{1 	ext{ mol}}$$

Question 13

You buy 2.60 lb of medium-sized nails, and a dozen nails weigh 0.150 lb. How many nails did you buy?

Accepted Answer

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

Question 14

Which of the following statements about the mole is correct?

Accepted Answer

A mole is a counting unit that represents $$6.022 	imes 10$$^{23}$$ entities, similar to how a dozen represents 12 items.

Question 15

If you have 0.75 moles of sulfur, how many grams of sulfur do you have? (Molar mass of S = 32.07 g/mol)

Accepted Answer

$$0.75 	imes 32.07 = 24.05 g$$

Question 16

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

Accepted Answer

To convert grams directly to moles without using molar mass.

Question 17

A copper mine produces 1.00 kg of copper. How many moles of copper is this? (Molar mass of Cu = 63.55 g/mol)

Accepted Answer

$$1000 \div 63.55 = 15.75$$ mol

Question 18

Which of the following best describes the significance of the atomic mass unit (amu)?

Accepted Answer

It is a unit used to express the mass of individual atoms, where 1 amu is defined as $$\frac{1}{12}$$ the mass of a carbon-12 atom.

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

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