Chemistry Practice Test: Stoichiometry and the Mole
- Stoichiometry and the Mole
- 15 questions
- 22 minutes
This test covers molar mass, converting between grams, moles and particles, and calculations from balanced equations. Use these rounded atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Cl = 35.5, Ca = 40, Fe = 56. Tip: always convert grams to moles before using the mole ratio from the equation.
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Question 1
Atomic masses: Ca = 40, C = 12, O = 16. What is the molar mass of calcium carbonate, CaCO3?
Correct answer: B
40 + 12 + 3 × 16 = 40 + 12 + 48 = 100 g/mol. The answer 68 g/mol counts only one oxygen atom, ignoring the subscript 3.
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Question 2
Atomic masses: C = 12, O = 16. How many moles are in 88 g of carbon dioxide, CO2?
Correct answer: D
The molar mass of CO2 is 12 + 2 × 16 = 44 g/mol, so 88 g ÷ 44 g/mol = 2.0 mol. Dividing 44 by 88 instead gives 0.5 mol, and using the mass of CO (28 g/mol) gives about 3.1 mol.
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Question 3
Avogadro's number is 6.02 × 1023. How many molecules are in 0.50 mol of water?
Correct answer: A
Number of molecules = moles × Avogadro's number = 0.50 × 6.02 × 1023 = 3.01 × 1023. The value 1.20 × 1024 comes from dividing by 0.50 instead of multiplying.
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Question 4
Propane burns in oxygen: ___C3H8 + ___O2 → ___CO2 + ___H2O. When the equation is balanced with the smallest whole numbers, what is the coefficient of O2?
Correct answer: C
Balance C first (3 CO2), then H (4 H2O). The right side then has 3 × 2 + 4 × 1 = 10 oxygen atoms, which takes 5 O2: C3H8 + 5O2 → 3CO2 + 4H2O. The answer 10 counts oxygen atoms, not O2 molecules.
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Question 5
Ammonia is made by the reaction N2 + 3H2 → 2NH3. How many moles of NH3 can form from 6 mol of H2 with enough N2?
Correct answer: B
The mole ratio is 3 mol H2 : 2 mol NH3, so 6 mol H2 × 2/3 = 4 mol NH3. Using the ratio upside down (6 × 3/2) gives 9 mol.
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Question 6
Hydrogen burns to form water: 2H2 + O2 → 2H2O. Atomic masses: H = 1, O = 16. What mass of water forms when 8.0 g of H2 reacts completely with oxygen?
Correct answer: D
8.0 g ÷ 2 g/mol = 4.0 mol H2. The ratio H2 : H2O is 2 : 2, so 4.0 mol of water forms, and 4.0 mol × 18 g/mol = 72 g. Using 1 g/mol for H2 gives 8 mol and 144 g.
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Question 7
Sodium reacts with chlorine: 2Na + Cl2 → 2NaCl. A reaction starts with 4.0 mol of Na and 1.5 mol of Cl2. What is the maximum amount of NaCl that can form?
Correct answer: A
4.0 mol Na would need 2.0 mol Cl2, but only 1.5 mol is present, so Cl2 is the limiting reactant. 1.5 mol Cl2 × 2 = 3.0 mol NaCl. The answer 4.0 mol assumes all the sodium reacts, which would need more chlorine.
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Question 8
Based on the amounts of reactants, a student calculates that 50.0 g of product should form. She actually collects 42.0 g. What is the percent yield?
Correct answer: C
Percent yield = actual ÷ theoretical × 100 = 42.0 ÷ 50.0 × 100 = 84%. Dividing the theoretical yield by the actual yield gives 119%, which is impossible for a real yield when the product is pure and dry.
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Question 9
Atomic masses: N = 14, H = 1, O = 16. What is the percent by mass of nitrogen in ammonium nitrate, NH4NO3?
Correct answer: B
The molar mass is 2 × 14 + 4 × 1 + 3 × 16 = 80 g/mol, and it contains 2 N atoms, 28 g. 28 ÷ 80 × 100 = 35.0%. Counting only one nitrogen gives 17.5%, and 60.0% is the percent of oxygen.
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Question 10
A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Atomic masses: C = 12, H = 1, O = 16. What is its empirical formula?
Correct answer: D
In 100 g: C = 40.0 ÷ 12 = 3.33 mol, H = 6.7 ÷ 1 = 6.7 mol, O = 53.3 ÷ 16 = 3.33 mol. Dividing by the smallest (3.33) gives 1 : 2 : 1, so CH2O. Using the mass percents directly as ratios, without converting to moles, leads to wrong formulas.
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Question 11
A compound has the empirical formula CH2O and a molar mass of 180 g/mol (C = 12, H = 1, O = 16). What is its molecular formula?
Correct answer: A
The empirical formula mass is 12 + 2 + 16 = 30 g/mol, and 180 ÷ 30 = 6, so every subscript is multiplied by 6: C6H12O6 (glucose). C6H6O6 forgets to multiply the hydrogen.
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Question 12
At STP (0 °C and 1 atm), 1 mol of any gas occupies 22.4 L. Atomic mass: O = 16. What is the mass of 11.2 L of oxygen gas, O2, at STP?
Correct answer: C
11.2 L ÷ 22.4 L/mol = 0.50 mol of O2, and 0.50 mol × 32 g/mol = 16 g. The answer 8.0 g uses 16 g/mol, the mass of a single oxygen atom, instead of O2.
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Question 13
Atomic masses: C = 12, H = 1. What is the mass of 3.01 × 1023 molecules of methane, CH4?
Correct answer: B
3.01 × 1023 ÷ 6.02 × 1023 = 0.500 mol, and the molar mass of CH4 is 12 + 4 = 16 g/mol, so the mass is 0.500 × 16 = 8.0 g. The answer 16 g is the mass of a full mole.
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Question 14
Iron is produced in a blast furnace: Fe2O3 + 3CO → 2Fe + 3CO2. Atomic masses: Fe = 56, O = 16. What mass of iron can be produced from 320 g of Fe2O3?
Correct answer: C
The molar mass of Fe2O3 is 2 × 56 + 3 × 16 = 160 g/mol, so 320 g is 2.0 mol. Each mole gives 2 mol Fe, so 4.0 mol × 56 g/mol = 224 g. Ignoring the 1 : 2 mole ratio gives 112 g.
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Question 15
When 10.0 g of calcium carbonate is heated, it decomposes completely: CaCO3 → CaO + CO2. The solid calcium oxide left behind has a mass of 5.6 g. What mass of carbon dioxide was released?
Correct answer: A
By the law of conservation of mass, the mass of the reactant equals the total mass of the products: 10.0 g − 5.6 g = 4.4 g of CO2. Adding the two masses (15.6 g) would mean mass was created, which cannot happen.
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