Chemistry Practice Test: The Mole and Molar Mass
- The Mole and Molar Mass
- 10 questions
- 15 minutes
This test covers molar mass and conversions between grams, moles and particles. Use these rounded atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, S = 32, Fe = 56, and Avogadro's number 6.02 × 1023. Tip: the mole is a counting unit, like a dozen, only much larger.
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Question 1
Atomic masses: H = 1, S = 32, O = 16. What is the molar mass of sulfuric acid, H2SO4?
Correct answer: D
2 × 1 + 32 + 4 × 16 = 2 + 32 + 64 = 98 g/mol. The value 97 g/mol counts only one hydrogen atom.
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Question 2
Atomic masses: Mg = 24, O = 16, H = 1. What is the molar mass of magnesium hydroxide, Mg(OH)2?
Correct answer: B
The subscript 2 applies to the whole OH group: 24 + 2 × (16 + 1) = 24 + 34 = 58 g/mol. The value 42 g/mol doubles only the hydrogen, as if the formula were MgOH2.
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Question 3
The molar mass of sodium hydroxide, NaOH, is 40 g/mol. What is the mass of 0.25 mol of NaOH?
Correct answer: A
Mass = moles × molar mass = 0.25 mol × 40 g/mol = 10 g. Dividing 40 by 0.25 instead gives 160 g, which is the wrong direction.
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Question 4
Atomic masses: Al = 27. How many moles of aluminum atoms are in 54 g of aluminum?
Correct answer: C
Moles = mass ÷ molar mass = 54 g ÷ 27 g/mol = 2.0 mol. Dividing 27 by 54 gives 0.50 mol, which flips the fraction.
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Question 5
How many atoms are in 2.0 mol of helium?
Correct answer: D
Each mole contains 6.02 × 1023 particles, so 2.0 × 6.02 × 1023 = 1.2 × 1024 atoms. 6.02 × 1023 is the number in only one mole.
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Question 6
How many moles of oxygen atoms are in 0.50 mol of calcium nitrate, Ca(NO3)2?
Correct answer: A
One formula unit has 2 × 3 = 6 oxygen atoms, so 0.50 mol contains 0.50 × 6 = 3.0 mol of O atoms. The answer 1.5 mol ignores the subscript 2 outside the parentheses.
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Question 7
Atomic masses: H = 1, C = 12, N = 14, O = 16. Which 10 g sample contains the most molecules?
Correct answer: B
Equal masses contain the most molecules when the molar mass is smallest. 10 g of H2 (2 g/mol) is 5 mol, while 10 g of CO2 (44 g/mol) is only about 0.23 mol.
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Question 8
Which statement about 1 mol of water and 1 mol of glucose is true?
Correct answer: C
A mole always means 6.02 × 1023 particles, but each substance has its own molar mass (water 18 g/mol, glucose 180 g/mol). The idea of equal volumes per mole applies only to gases, and neither substance is a gas at room temperature.
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Question 9
Atomic masses: Fe = 56. What is the mass of 1.204 × 1024 atoms of iron?
Correct answer: A
1.204 × 1024 ÷ 6.02 × 1023 = 2.0 mol, and 2.0 mol × 56 g/mol = 112 g. 56 g is the mass of only one mole.
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Question 10
Epsom salt is the hydrate MgSO4·7H2O. Atomic masses: Mg = 24, S = 32, O = 16, H = 1. What percent of its mass is water?
Correct answer: C
The molar mass is 120 + 7 × 18 = 246 g/mol, and the water is 126 g, so 126 ÷ 246 × 100 = 51.2%. 48.8% is the percent of MgSO4, and 7.3% counts only one water molecule.
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