Rucete ✏ AP Chemistry In a Nutshell
5. Stoichiometry — Practice Questions 2
This chapter explores the quantitative relationships in chemical reactions, including mole ratios, percent composition, limiting reagents, and molarity.
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(Multiple Choice — Click to Reveal Answer)
1. What is the molar mass of (NH₄)₂SO₄?
(A) 96.0 g/mol
(B) 118.1 g/mol
(C) 132.1 g/mol
(D) 150.2 g/mol
Answer
(D) — (2 × 14.0) + (8 × 1.0) + (32.1) + (4 × 16.0) = 150.2 g/mol.
2. How many moles are in 25.0 g of NaOH (molar mass = 40.0 g/mol)?
(A) 0.625 mol
(B) 1.00 mol
(C) 1.25 mol
(D) 1.60 mol
Answer
(A) — 25.0 ÷ 40.0 = 0.625 mol.
3. Which quantity represents Avogadro’s number?
(A) 6.02 × 10²²
(B) 1.00 mol
(C) 6.02 × 10²³
(D) 1.66 × 10⁻²⁴
Answer
(C) — Avogadro’s number = 6.02 × 10²³ particles/mol.
4. Which is true about molar mass?
(A) It varies with temperature
(B) It is the same for all isotopes
(C) It is the mass of 1 mole of a substance
(D) It must be measured in mL
Answer
(C) — Molar mass is defined as the mass in grams of one mole of a substance.
5. What volume will 2.00 mol of CO₂ occupy at STP?
(A) 22.4 L
(B) 44.8 L
(C) 11.2 L
(D) 33.6 L
Answer
(B) — 2.00 mol × 22.4 L/mol = 44.8 L.
6. What is the percentage of oxygen in Ca(NO₃)₂?
(A) 16.0%
(B) 32.0%
(C) 48.0%
(D) 58.5%
Answer
(D) — Oxygen: 96.0 g in 164.1 g → (96/164.1) × 100 = 58.5%.
7. A compound contains 12.0 g of C and 2.0 g of H. What is its empirical formula?
(A) CH
(B) C₂H₄
(C) CH₄
(D) C₆H₁₂
Answer
(C) — Convert: C: 1 mol, H: 2 mol → simplest ratio = CH₂.
8. How many atoms are there in 0.25 mol of Zn?
(A) 1.50 × 10²³
(B) 6.02 × 10²³
(C) 3.01 × 10²³
(D) 4.50 × 10²³
Answer
(C) — 0.25 × 6.02 × 10²³ = 1.505 × 10²³ atoms.
9. How many moles are present in 20.0 L of O₂ at STP?
(A) 0.89 mol
(B) 1.25 mol
(C) 1.78 mol
(D) 2.24 mol
Answer
(C) — 20.0 ÷ 22.4 = 0.8928 mol.
10. What is the limiting reagent when 4.0 mol of H₂ reacts with 2.0 mol of O₂?
2H₂ + O₂ → 2H₂O
(A) H₂
(B) O₂
(C) H₂O
(D) None
Answer
(B) — H₂ needs 2 mol of O₂ for 4 mol; only 2 mol O₂ is available.
11. What is the mass of 0.200 mol of CaCl₂ (molar mass = 110.98 g/mol)?
(A) 18.2 g
(B) 22.0 g
(C) 27.7 g
(D) 33.0 g
Answer
(C) — 0.200 mol × 110.98 g/mol = 22.2 g.
12. Which unit is used to express molar mass?
(A) mol
(B) g/mol
(C) L/mol
(D) g/L
Answer
(B) — Molar mass is measured in grams per mole.
13. What is the total number of oxygen atoms in one formula unit of Al₂(SO₄)₃?
(A) 4
(B) 6
(C) 9
(D) 12
Answer
(D) — 3 sulfate ions × 4 oxygen atoms = 12 oxygen atoms.
14. If 1.00 mol of CH₄ reacts with 2.00 mol of O₂, how many moles of CO₂ are produced?
CH₄ + 2O₂ → CO₂ + 2H₂O
(A) 0.50 mol
(B) 1.00 mol
(C) 1.50 mol
(D) 2.00 mol
Answer
(B) — 1 mol CH₄ gives 1 mol CO₂.
15. What is the percent composition of hydrogen in H₂O?
(A) 11.1%
(B) 16.0%
(C) 33.3%
(D) 66.7%
Answer
(A) — H₂O molar mass = 18 g/mol → H = 2/18 = 11.1%.
16. How many moles are in 5.60 L of a gas at STP?
(A) 0.25 mol
(B) 0.50 mol
(C) 0.75 mol
(D) 1.00 mol
Answer
(A) — 5.60 ÷ 22.4 = 0.25 mol.
17. What is the correct mole ratio between N₂ and NH₃ in the equation:
N₂ + 3H₂ → 2NH₃?
(A) 1:2
(B) 2:3
(C) 3:2
(D) 2:1
Answer
(A) — 1 mol N₂ gives 2 mol NH₃.
18. If the actual yield is 40.0 g and the theoretical yield is 50.0 g, what is the percent yield?
(A) 60.0%
(B) 70.0%
(C) 80.0%
(D) 90.0%
Answer
(C) — (40.0 / 50.0) × 100 = 80.0%
19. What is the number of molecules in 2.00 mol of CO₂?
(A) 1.20 × 10²³
(B) 2.00 × 10²³
(C) 6.02 × 10²³
(D) 1.20 × 10²⁴
Answer
(D) — 2 × 6.02 × 10²³ = 1.204 × 10²⁴.
20. Which compound has the highest percentage of nitrogen by mass?
(A) NH₃
(B) NO
(C) N₂O
(D) N₂
Answer
(D) — N₂ is 100% nitrogen by mass.
21. What is the empirical formula for a compound with a 1:2 molar ratio of carbon to hydrogen?
(A) CH₂
(B) C₂H₄
(C) C₃H₆
(D) CH
Answer
(A) — Simplest whole-number ratio of 1:2 is CH₂.
22. What is the mass of 0.050 mol of CO₂?
(A) 1.10 g
(B) 2.20 g
(C) 3.30 g
(D) 4.40 g
Answer
(D) — 0.050 mol × 44.0 g/mol = 2.20 g.
23. How many molecules are in 0.100 mol of water?
(A) 6.02 × 10²²
(B) 1.20 × 10²³
(C) 6.02 × 10²³
(D) 3.01 × 10²³
Answer
(B) — 0.100 mol × 6.02 × 10²³ = 6.02 × 10²² molecules.
24. Which element has the highest percentage by mass in H₂SO₄?
(A) H
(B) S
(C) O
(D) All equal
Answer
(C) — O = 64 g out of 98 g = 65.3%.
25. What is the mass of 3.00 mol of Fe₂O₃ (molar mass = 159.7 g/mol)?
(A) 159.7 g
(B) 319.4 g
(C) 479.1 g
(D) 639.0 g
Answer
(C) — 3 × 159.7 = 479.1 g.
26. A reaction yields 15.0 g of product from a theoretical yield of 20.0 g. What is the percent yield?
(A) 65.0%
(B) 70.0%
(C) 75.0%
(D) 80.0%
Answer
(C) — (15.0 ÷ 20.0) × 100 = 75.0%
27. In the combustion of propane, what is the mole ratio between O₂ and H₂O?
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
(A) 5:4
(B) 3:4
(C) 4:5
(D) 1:1
Answer
(A) — 5 moles O₂ produces 4 moles H₂O → ratio = 5:4.
28. Which compound has the greatest percent composition of oxygen?
(A) CO₂
(B) H₂O
(C) CH₃COOH
(D) NaNO₃
Answer
(A) — CO₂ is ~72.7% oxygen by mass (32/44).
29. How many grams of NaCl are required to make 250.0 mL of 0.400 M solution? (Molar mass = 58.44 g/mol)
(A) 2.92 g
(B) 5.86 g
(C) 7.31 g
(D) 11.7 g
Answer
(B) — mol = M × L = 0.400 × 0.250 = 0.100 mol → 0.100 × 58.44 = 5.84 g.
30. Which statement is true about limiting reactants?
(A) It is always the reactant with the smallest mass
(B) It determines the maximum product formed
(C) It is the reactant in greatest excess
(D) It forms the fewest types of products
Answer
(B) — The limiting reactant limits the amount of product that can form.
31. What is the empirical formula of a compound with 43.6% Na, 15.2% C, and 41.2% O by mass?
(A) NaCO₂
(B) Na₂CO₃
(C) Na₂C₂O₄
(D) NaCO₃
Answer
(B) — Na: 43.6/23 ≈ 1.90, C: 15.2/12 = 1.27, O: 41.2/16 = 2.58 → ratio ≈ 2:1:3 = Na₂CO₃.
32. When 2.00 mol of A reacts with 3.00 mol of B to form C via A + 2B → C, what is the limiting reactant?
(A) A
(B) B
(C) C
(D) Cannot be determined
Answer
(B) — A needs 4 mol B, but only 3 mol B available → B is limiting.
33. In a decomposition reaction, 10.0 g of a compound produces 4.00 g of product A and 5.00 g of product B. What is the percent yield if the theoretical yield was 10.5 g total?
(A) 76.2%
(B) 85.7%
(C) 90.5%
(D) 95.2%
Answer
(C) — Actual yield = 9.00 g → (9.00 / 10.5) × 100 = 85.7%.
34. A student wants to prepare 0.200 mol of HCl solution at 1.00 M concentration. What volume is needed?
(A) 100.0 mL
(B) 150.0 mL
(C) 200.0 mL
(D) 250.0 mL
Answer
(C) — V = mol / M = 0.200 / 1.00 = 0.200 L = 200.0 mL.
35. How many grams of H₂ are needed to react completely with 16.0 g of O₂?
2H₂ + O₂ → 2H₂O
(A) 1.00 g
(B) 2.00 g
(C) 4.00 g
(D) 8.00 g
Answer
(C) — 16.0 g O₂ = 1.00 mol → needs 2.00 mol H₂ = 2.00 × 2.0 = 4.00 g.
36. Explain the concept of a limiting reactant in a chemical reaction.
Answer
The limiting reactant is the reactant that is completely used up first in a chemical reaction, thus limiting the amount of product that can be formed.
37. A 2.00 g sample of magnesium reacts with hydrochloric acid. Write a balanced chemical equation and calculate the number of moles of hydrogen gas produced.
Answer
Equation: Mg + 2HCl → MgCl₂ + H₂ Calculation: 2.00 g Mg × (1 mol / 24.3 g) = 0.0823 mol Mg → 0.0823 mol H₂ produced.
38. Describe the steps to determine the empirical formula of a compound from its percent composition.
Answer
1. Convert percentages to grams (assume 100 g sample). 2. Convert grams to moles. 3. Divide all mole values by the smallest mole. 4. Multiply to get whole numbers if needed.
39. Calculate the molar mass of Ca₃(PO₄)₂.
Answer
310.18 g/mol — 3 Ca (40.08) + 2 P (30.97) + 8 O (16.00) = 310.18 g/mol.
40. How many moles of oxygen gas are needed to completely react with 5.00 mol of methane (CH₄)? CH₄ + 2O₂ → CO₂ + 2H₂O
Answer
10.0 mol — 1 mol CH₄ requires 2 mol O₂ → 5.00 × 2 = 10.0 mol.
41. A 0.300 mol solution of NaOH requires 25.0 mL to neutralize an HCl solution. What is the concentration of HCl if 50.0 mL was used?
Answer
0.150 M — M₁V₁ = M₂V₂ → (0.300)(25.0) = M × 50.0 → M = 0.150 M.
42. Determine the number of particles in 0.750 mol of a substance.
Answer
4.52 × 10²³ particles — 0.750 mol × 6.02 × 10²³ = 4.515 × 10²³.
43. Explain why the actual yield in a chemical reaction is usually less than the theoretical yield.
Answer
Due to incomplete reactions, side reactions, loss during purification or transfer, and measurement errors.
44. A student dissolves 5.00 g of NaCl in 100.0 mL of water. What is the molarity of the solution?
Answer
0.855 M — mol = 5.00 / 58.44 = 0.0855 mol → M = 0.0855 / 0.100 = 0.855 M.
45. What is the percent composition of carbon in C₂H₆O?
Answer
52.1% — (2 × 12.01) / (46.08) × 100 = 52.1%
46. Describe how the mole concept connects mass, particles, and volume at STP.
Answer
1 mol = molar mass in grams = 6.02 × 10²³ particles = 22.4 L gas at STP.
47. A reaction produced 2.00 mol of CO₂. What volume will this occupy at STP?
Answer
44.8 L — 2.00 mol × 22.4 L/mol = 44.8 L.
48. Explain the difference between an empirical formula and a molecular formula.
Answer
Empirical formula shows the simplest whole-number ratio of atoms. Molecular formula shows the actual number of each atom in the molecule.
49. How many grams of aluminum are needed to react with 1.00 mol of Fe₂O₃? 2Al + Fe₂O₃ → 2Fe + Al₂O₃
Answer
54.0 g — 2 mol Al reacts with 1 mol Fe₂O₃ → 2 × 27.0 = 54.0 g.
50. A compound is found to have a molar mass of 180 g/mol and an empirical formula of CH₂O. What is its molecular formula?
Answer
C₆H₁₂O₆ — CH₂O = 30 g/mol → 180 / 30 = 6 → multiply empirical formula by 6.