Rucete ✏ AP Chemistry In a Nutshell
5. Stoichiometry — Practice Questions 3
This chapter introduces stoichiometric calculations using dimensional analysis, focusing on limiting reactants, percent composition, empirical formulas, titrations, and gas laws.
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(Multiple Choice — Click to Reveal Answer)
1. Which of the following is equivalent to one mole of H₂O?
(A) 18.0 molecules
(B) 6.02 × 10²³ molecules
(C) 18.0 grams of hydrogen
(D) 6.02 × 10²³ atoms of hydrogen
Answer
(B) — One mole of any substance contains Avogadro’s number of particles (6.02 × 10²³).
2. What is the molar mass of Ca(NO₃)₂?
(A) 40.1 g/mol
(B) 62.0 g/mol
(C) 164.1 g/mol
(D) 102.1 g/mol
Answer
(C) — 40.1 (Ca) + 2 × [14.0 (N) + 3 × 16.0 (O)] = 164.1 g/mol.
3. Which measurement is most useful when converting from grams to moles?
(A) Molarity
(B) Molar mass
(C) Volume
(D) Temperature
Answer
(B) — Molar mass is used to convert between grams and moles.
4. Which equation is used to determine the number of particles in a sample?
(A) M = mol/L
(B) moles = grams/molar mass
(C) particles = moles × 6.02 × 10²³
(D) PV = nRT
Answer
(C) — The number of particles is calculated by multiplying moles by Avogadro’s number.
5. What is the percent composition of carbon in CH₄?
(A) 25.0%
(B) 50.0%
(C) 75.0%
(D) 12.0%
Answer
(C) — (12.0 / 16.0) × 100 = 75.0%
6. A sample contains 0.500 mol of NaCl. How many Na⁺ ions are present?
(A) 3.01 × 10²³
(B) 1.00 mol
(C) 0.500 mol
(D) 6.02 × 10²³
Answer
(A) — Each mole of NaCl yields one Na⁺ ion: 0.500 mol × 6.02 × 10²³ = 3.01 × 10²³ ions.
7. How many liters of hydrogen gas at STP are in 2.00 mol of H₂?
(A) 11.2 L
(B) 22.4 L
(C) 44.8 L
(D) 1.00 L
Answer
(C) — 2.00 mol × 22.4 L/mol = 44.8 L
8. What is the empirical formula of a compound with 40% C, 6.7% H, and 53.3% O?
(A) C₂H₄O₂
(B) CH₂O
(C) CH₄O
(D) C₂H₂O₂
Answer
(B) — The simplest ratio is 1:2:1 → CH₂O.
9. Which of the following is a typical step in stoichiometric calculations?
(A) Determine valence electrons
(B) Use PV = nRT
(C) Balance the chemical equation
(D) Draw Lewis structures
Answer
(C) — Stoichiometry requires a balanced chemical equation.
10. Which of the following conversions would require Avogadro’s number?
(A) moles to liters
(B) grams to moles
(C) particles to moles
(D) liters to grams
Answer
(C) — Avogadro’s number is used to convert between particles and moles.
11. What is the percent by mass of hydrogen in H₂O?
(A) 11.1%
(B) 22.2%
(C) 88.9%
(D) 33.3%
Answer
(A) — (2.02 / 18.02) × 100 = 11.1%
12. If a compound has a molar mass of 90 g/mol and empirical formula CH₂O, what is its molecular formula?
(A) CH₂O
(B) C₂H₄O₂
(C) C₃H₆O₃
(D) C₄H₈O₄
Answer
(C) — CH₂O = 30 → 90 / 30 = 3 → (CH₂O)₃ = C₃H₆O₃
13. Which formula represents an empirical formula?
(A) C₆H₁₂O₆
(B) CH₂O
(C) C₂H₆
(D) C₄H₁₀
Answer
(B) — CH₂O is the simplest whole number ratio.
14. The molar volume of any gas at STP is:
(A) 1.00 L
(B) 11.2 L
(C) 22.4 L
(D) 44.8 L
Answer
(C) — At STP, 1 mol of gas occupies 22.4 L.
15. What is the mass of 0.250 mol of CO₂?
(A) 11.0 g
(B) 22.0 g
(C) 44.0 g
(D) 66.0 g
Answer
(A) — 0.250 mol × 44.0 g/mol = 11.0 g
16. What is the mole ratio of H₂ to H₂O in the reaction 2H₂ + O₂ → 2H₂O?
(A) 1:1
(B) 2:1
(C) 2:2
(D) 1:2
Answer
(A) — 2 moles H₂ produce 2 moles H₂O → ratio = 1:1
17. How many moles are in 10.0 g of helium?
(A) 1.00 mol
(B) 2.50 mol
(C) 5.00 mol
(D) 10.0 mol
Answer
(B) — 10.0 g / 4.00 g/mol = 2.50 mol
18. What is the molarity of a solution made by dissolving 0.500 mol NaCl in 250 mL of solution?
(A) 0.200 M
(B) 0.500 M
(C) 2.00 M
(D) 4.00 M
Answer
(C) — 0.500 mol / 0.250 L = 2.00 M
19. In a titration, the point at which moles of acid equal moles of base is called:
(A) Saturation point
(B) Halfway point
(C) End point
(D) Equivalence point
Answer
(D) — The equivalence point is where acid and base are present in stoichiometrically equal amounts.
20. What type of reaction is used in titration calculations?
(A) Decomposition
(B) Combustion
(C) Neutralization
(D) Oxidation
Answer
(C) — Titration is based on acid-base neutralization reactions.
21. The ratio of actual yield to theoretical yield is known as:
(A) Percent yield
(B) Percent error
(C) Molar ratio
(D) Stoichiometric factor
Answer
(A) — Percent yield = (actual / theoretical) × 100
22. What is the molar mass of Fe₂O₃?
(A) 159.7 g/mol
(B) 56.0 g/mol
(C) 103.9 g/mol
(D) 71.9 g/mol
Answer
(A) — (2 × 55.85) + (3 × 16.00) = 159.7 g/mol
23. What is the primary purpose of using dimensional analysis in stoichiometry?
(A) To balance equations
(B) To visualize molecules
(C) To ensure correct unit conversion
(D) To predict molecular geometry
Answer
(C) — Dimensional analysis ensures units are consistent and cancel properly.
24. Which quantity remains constant in a closed reaction system?
(A) Mass
(B) Volume
(C) Pressure
(D) Molarity
Answer
(A) — According to the Law of Conservation of Mass, total mass remains constant.
25. What is the percent by mass of oxygen in H₂O?
(A) 33.3%
(B) 50.0%
(C) 66.6%
(D) 88.9%
Answer
(D) — (16.00 / 18.02) × 100 = 88.9%
26. When 4.00 g of hydrogen gas reacts with excess oxygen, how many grams of water are formed? (2H₂ + O₂ → 2H₂O)
(A) 18.0 g
(B) 32.0 g
(C) 36.0 g
(D) 72.0 g
Answer
(C) — 4.00 g H₂ = 2.00 mol → forms 2.00 mol H₂O → 2.00 × 18.0 = 36.0 g
27. In a reaction, 0.500 mol of A reacts with 0.400 mol of B to form AB₂. Which is the limiting reactant? (A + 2B → AB₂)
(A) A
(B) B
(C) AB₂
(D) Cannot be determined
Answer
(B) — 0.500 mol A would require 1.00 mol B, but only 0.400 mol B is present.
28. A student dissolves 10.0 g of NaOH in water and makes 250 mL of solution. What is the molarity? (Molar mass of NaOH = 40.0 g/mol)
(A) 0.10 M
(B) 0.40 M
(C) 1.00 M
(D) 2.00 M
Answer
(D) — 10.0 g / 40.0 = 0.250 mol → 0.250 mol / 0.250 L = 1.00 M
29. What is the molecular formula of a compound with empirical formula CH₂ and molar mass 56 g/mol?
(A) CH₂
(B) C₂H₄
(C) C₃H₆
(D) C₄H₈
Answer
(C) — CH₂ = 14 g/mol → 56 / 14 = 4 → (CH₂)₄ = C₄H₈
30. A gas occupies 10.0 L at STP. How many grams of oxygen gas (O₂) does this represent? (Molar mass = 32.0 g/mol)
(A) 10.0 g
(B) 14.3 g
(C) 16.0 g
(D) 22.4 g
Answer
(B) — 10.0 L / 22.4 L = 0.446 mol → 0.446 × 32.0 = 14.3 g
31. Which of the following compounds contains the highest percent nitrogen by mass?
(A) NH₃
(B) N₂O
(C) NO₂
(D) N₂H₄
Answer
(A) — N is 14.0, H₃ = 3.0 → 14/17 = 82.4%, which is highest.
32. In a titration, it took 30.0 mL of 0.100 M NaOH to neutralize 20.0 mL of HCl. What is the concentration of HCl?
(A) 0.050 M
(B) 0.100 M
(C) 0.150 M
(D) 0.200 M
Answer
(D) — M₁V₁ = M₂V₂ → 0.100 × 30.0 = M × 20.0 → M = 0.150 M
33. 2.00 mol of CaCl₂ dissolves in water. How many chloride ions are present?
(A) 6.02 × 10²³
(B) 1.20 × 10²⁴
(C) 2.00 mol
(D) 4.00 mol
Answer
(B) — Each CaCl₂ gives 2 Cl⁻ → 2.00 mol × 2 = 4.00 mol × 6.02 × 10²³ = 1.20 × 10²⁴
34. In the reaction N₂ + 3H₂ → 2NH₃, how many liters of H₂ at STP are required to produce 8.96 L of NH₃?
(A) 4.48 L
(B) 8.96 L
(C) 13.4 L
(D) 20.1 L
Answer
(C) — 2 NH₃ requires 3 H₂ → 8.96 L × (3/2) = 13.4 L
35. A reaction has a theoretical yield of 24.0 g and actual yield of 20.0 g. What is the percent yield?
(A) 80.0%
(B) 83.3%
(C) 90.0%
(D) 95.0%
Answer
(A) — (20.0 / 24.0) × 100 = 83.3%
36. Explain how you would determine the limiting reactant in a chemical reaction using stoichiometry.
Answer
Convert the amounts of each reactant to moles of product. The reactant that produces the lesser amount of product is the limiting reactant.
37. Describe the steps to calculate percent composition from a molecular formula.
Answer
1. Find molar mass of the compound. 2. Divide the mass of each element by the total molar mass. 3. Multiply by 100 to get the percent.
38. A compound is 92.3% carbon and 7.7% hydrogen. What is its empirical formula?
Answer
C₃H — 92.3 g C = 7.69 mol, 7.7 g H = 7.63 mol → ratio ~ 1:1 → C₃H
39. What volume of 0.500 M H₂SO₄ is required to neutralize 25.0 mL of 1.00 M NaOH?
Answer
25.0 mL — H₂SO₄ has 2 H⁺ per molecule → M₁V₁ = 2 × M₂V₂ → (0.500)V = 1.00 × 25.0 → V = 25.0 mL
40. How many grams of CO₂ are produced from 10.0 g of CH₄? CH₄ + 2O₂ → CO₂ + 2H₂O
Answer
27.5 g — 10.0 g CH₄ = 0.625 mol → 0.625 mol CO₂ × 44.0 = 27.5 g
41. Define theoretical yield and explain its importance in stoichiometry.
Answer
The theoretical yield is the maximum amount of product that can be formed based on the limiting reactant. It helps determine reaction efficiency and percent yield.
42. What is the volume at STP of 1.50 mol of nitrogen gas?
Answer
33.6 L — 1.50 mol × 22.4 L/mol = 33.6 L
43. Calculate the molarity of a solution prepared by dissolving 25.0 g of KCl (MM = 74.6 g/mol) in 500 mL of water.
Answer
0.670 M — mol = 25.0 / 74.6 = 0.335 mol → 0.335 / 0.500 L = 0.670 M
44. How many atoms are in 3.00 mol of aluminum?
Answer
1.81 × 10²⁴ atoms — 3.00 mol × 6.02 × 10²³ = 1.806 × 10²⁴
45. A compound contains 58.8% C, 9.8% H, and 31.4% O. What is the empirical formula?
Answer
C₂H₄O — convert % to mol: 58.8 g C = 4.9 mol, 9.8 g H = 9.8 mol, 31.4 g O = 2.0 mol → simplest ratio = C₂H₄O
46. Explain why balancing chemical equations is essential before doing stoichiometric calculations.
Answer
Balancing ensures the law of conservation of mass is obeyed and the correct mole ratios are used in calculations.
47. What is the actual yield if the theoretical yield is 30.0 g and the percent yield is 80.0%?
Answer
24.0 g — (80.0 / 100) × 30.0 = 24.0 g
48. A gas occupies 5.00 L at STP. How many moles is this?
Answer
0.223 mol — 5.00 / 22.4 = 0.223 mol
49. Define molarity and explain how it is used in stoichiometry.
Answer
Molarity (M) is moles of solute per liter of solution. It's used to relate solution volume to amount of substance in reactions.
50. If 0.250 mol of CaCl₂ is dissolved in 1.00 L of water, what is the concentration of Cl⁻ ions?
Answer
0.500 M — 1 mol CaCl₂ → 2 mol Cl⁻ → 0.250 mol gives 0.500 mol Cl⁻ in 1.00 L = 0.500 M