The Mole Concept: Quiz

The Mole Concept: Quiz

7 questions

1. Question 1EasyHow many particles are in one mole of a substance?

   A$6.022 \times 10^{23}$B$6.022 \times 10^{-23}$C12DIt depends on the substance
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   Answer: $6.022 \times 10^{23}$

   A mole is a fixed counting number, like a dozen. One mole of any substance contains $6.022 \times 10^{23}$ particles. The mass of a mole depends on the substance, but the number of particles does not.

2. Question 2EasyWhat is the molar mass of carbon dioxide, $\ce{CO2}$? (C = 12.01, O = 16.00)

   A28.01 g/molB32.00 g/molC44.01 g/molD60.01 g/mol
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   Answer: 44.01 g/mol

   One carbon atom plus two oxygen atoms: $12.01 + 2(16.00) = 44.01$ g/mol. The answer 28.01 g/mol forgets the subscript 2 (it is the molar mass of CO).

3. Question 3MediumHow many moles are in 9.00 g of water, $\ce{H2O}$? (M = 18.02 g/mol)

   A0.499 molB2.00 molC9.00 molD162 mol
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   Answer: 0.499 mol

   $n = \dfrac{m}{M} = \dfrac{9.00\ \text{g}}{18.02\ \text{g/mol}} = 0.499$ mol. The answer 162 mol comes from multiplying instead of dividing. Check the units: g ÷ (g/mol) gives mol.

4. Question 4MediumWhat is the mass of 0.500 mol of sodium chloride, $\ce{NaCl}$? (M = 58.44 g/mol)

   A0.00856 gB29.2 gC58.4 gD117 g
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   Answer: 29.2 g

   $m = n \times M = 0.500\ \text{mol} \times 58.44\ \text{g/mol} = 29.2$ g. The answer 117 g comes from dividing 58.44 by 0.500 instead of multiplying.

5. Question 5MediumHow many oxygen atoms are in 1.00 mol of oxygen gas, $\ce{O2}$?

   A$3.011 \times 10^{23}$B$6.022 \times 10^{23}$C$1.204 \times 10^{24}$D16.00
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   Answer: $1.204 \times 10^{24}$

   1.00 mol of $\ce{O2}$ contains $6.022 \times 10^{23}$ molecules. Each molecule has 2 oxygen atoms, so there are $2 \times 6.022 \times 10^{23} = 1.204 \times 10^{24}$ atoms.

6. Question 6HardWhich sample contains more atoms: 1 mol of helium, $\ce{He}$, or 1 mol of hydrogen gas, $\ce{H2}$?

   A1 mol of $\ce{He}$B1 mol of $\ce{H2}$CThey contain the same number of atomsDIt cannot be decided without their masses
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   Answer: 1 mol of $\ce{H2}$

   Both samples contain the same number of particles, $6.022 \times 10^{23}$. Helium particles are single atoms, but each $\ce{H2}$ molecule contains two atoms. So 1 mol of $\ce{H2}$ contains twice as many atoms.

7. Question 7HardSince 2019, how is the mole defined?

   ABy fixing the numerical value of the Avogadro constant exactlyBAs the number of atoms in exactly 12 g of carbon-12CAs the volume occupied by a gas at standard conditionsDAs the mass of one hydrogen atom in grams
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   Answer: By fixing the numerical value of the Avogadro constant exactly

   In the 2019 revision of the SI, the Avogadro constant was fixed at exactly $6.022\,140\,76 \times 10^{23}$ mol⁻¹. The carbon-12 definition was used before 2019, and many textbooks still mention it.