General Chemistry I — practiceOwulade Past Questions

University of Ilorin CHM101: 49 questions.

Topics: Elementary thermochemistry, Chemical equations and stoichiometry, Radioactivity

1. Which statement is true about 'order' and 'molecularity' of reactions?
   1. Order is usually constant while molecularity varies.
   2. Molecularity is usually twice the order.
   3. Order can only be determined from experiments.
   4. Molecularity is the same as order in all cases.
2. Which statement is correct about the decay constant in radioactive decays?
   1. The value is not constant throughout the reaction.
   2. The value is not affected by any external influence.
   3. The value is affected by temperature and pressure only.
   4. The value cannot be determined.
3. A radioactive decay is always considered as:
   1. Zero-order reaction
   2. First-order reaction
   3. Both zero and first-order
   4. Second-order reaction
4. Assume the reaction: A → Products, is a simple first-order decay. If the amount of A drops from an initial value at t = 0 of 0.5000 mol to 0.0625 mol in exactly 1 h, what is the rate constant k and half-life for the reaction?
   1. k = 0.0116 s⁻¹, t₁/₂ = 60 s
   2. k = 0.00193 s⁻¹, t₁/₂ = 360 s
   3. k = 0.693 h⁻¹, t₁/₂ = 1 h
   4. k = 2.08 × 10⁻³ s⁻¹, t₁/₂ = 333 s
5. The half-life of a substrate in a certain enzyme-catalyzed first-order reaction is 138 s. How long is required for the concentration of substrate to fall from 1.28 mmol dm⁻³ to 0.040 mmol dm⁻³?
   1. 690 s
   2. 960 s
   3. 609 s
   4. 906 s
6. Zinc-65 has a half-life of 245 days. (a) What % of the original activity remains after 100 days? (b) How much time is required for the activity to reduce to 5% of the initial activity?
   1. (a) 73.5%, (b) 1063 days
   2. (a) 63.5%, (b) 1063 days
   3. (a) 53.5%, (b) 1000 days
   4. (a) 83.5%, (b) 963 days
7. Determine the half-life of a zero-order reaction if the original concentration of the reactant is 1.2 mol dm⁻³ and k₀ is 0.00625 mol dm⁻³ s⁻¹.
   1. 625 s
   2. 69 s
   3. 96 s
   4. 62500 s
8. The gas-phase reaction: 2NO₂ + O₃ → N₂O₅ + O₂, has the rate constant k = 2.05 × 10⁴ mol dm⁻³ s⁻¹ at 300 K. What is the order of the reaction?
   1. 1
   2. 2
   3. 0
   4. 3
9. Calculate the half-life for N₂O₅ at 25 °C and the fraction decomposed after 5 hours; k = 3.38 × 10⁻⁵ s⁻¹.
   1. 20800 s, 0.378
   2. 20600 s, 0.677
   3. 20500 s, 0.392
   4. 20789 s, 0.396
10. If the rate constant for the isomerization of HNC to HCN is 4.403 × 10⁻⁴ s⁻¹, what mass of HNC remains after 1.50 hr if a 1.000-gram sample was present initially?
    1. 0.419 g
    2. 0.00419 g
    3. 0.0419 g
    4. 0.000419 g
11. The half-life of a first-order reaction is 693 s. The initial concentration is 0.1 mol dm⁻³. Calculate the rate constant and the concentration after 5 minutes.
    1. 0.001 s⁻¹, 0.074 mol dm⁻³
    2. 0.01 s⁻¹, 0.074 mol dm⁻³
    3. 0.1 s⁻¹, 0.0074 mol dm⁻³
    4. 0.01 s⁻¹, 0.74 mol dm⁻³
12. The gas-phase decomposition of N2O5 is first-order. The rate constant at 337.6 K is 5.12 × 10⁻³ s⁻¹. If the partial pressure of N2O5 is 0.500 atm at t = 0, find its partial pressure at t = 60.0 s.
    1. 0.278 atm
    2. 0.368 atm
    3. 0.728 atm
    4. 0.638 atm
13. The half-life of 235U is 7.1 × 10⁸ years. Find the first-order rate constant.
    1. 9.8 × 10⁻¹⁰ yr⁻¹
    2. 8.9 × 10⁻¹⁰ yr⁻¹
    3. 9.8 × 10⁻¹⁰ day⁻¹
    4. 8.9 × 10⁻¹⁰ day⁻¹
14. A zero-order reaction has k = 0.0150 mol L⁻¹ s⁻¹. If [A]₀ = 1.000 mol L⁻¹, find [A] after 5.00 s.
    1. 0.529 mol L⁻¹
    2. 0.259 mol L⁻¹
    3. 0.925 mol L⁻¹
    4. 0.0259 mol L⁻¹
15. A first-order reaction is 40.0% complete in 185 s. Find k and t₁/₂ if [A]₀ = 0.100 mol L⁻¹.
    1. 0.00627 s⁻¹, 251 s
    2. 0.00726 s⁻¹, 152 s
    3. 0.00672 s⁻¹, 152 s
    4. 0.00276 s⁻¹, 251 s
16. The reaction SO2Cl2 → SO2 + Cl2 has k = 2.2 × 10⁻⁵ s⁻¹ at 320°C. Find t₁/₂.
    1. 3.2 × 10⁴ s
    2. 2.3 × 10⁴ s
    3. 4.6 × 10⁴ s
    4. 6.4 × 10⁴ s
17. Potassium-40 (0.0118% in KCl) has t₁/₂ = 1.27 × 10⁹ years. Calculate disintegrations per second in 1 g KCl.
    1. 1.57 s⁻¹
    2. 157 s⁻¹
    3. 0.157 s⁻¹
    4. 15.7 s⁻¹
18. Which statements are correct for zero-order reactions? (i) Rate is time-independent (ii) Reactants don't react (iii) Excess reactant (iv) [A] decreases with time.
    1. (i) & (iv)
    2. (ii) & (iii)
    3. (i) & (iv)
    4. (ii) & (iv)
19. A radioactive decay is always considered as:
    1. Zero-order
    2. First-order
    3. Both
    4. Second-order
20. For reaction A → B + C, [A] halves in 1 h (4M→2M) and again in 0.5 h (2M→1M). The order is:
    1. Second
    2. First
    3. Third
    4. Zero
21. Zero-order reactions have the same unit for rate constant and:
    1. Time
    2. Reactant concentration
    3. Rate
    4. None
22. For a zero-order reaction with k = 2 × 10⁻² mol/L s and [A] = 0.5 M after 25 s, find [A]₀.
    1. 1 mol/L
    2. 0.1 mol/L
    3. 0.01 mol/L
    4. 0.001 mol/L
23. For a zero-order reaction, t₁/₂ depends on [A]₀ as:
    1. [A]₀
    2. [A]₀¹
    3. [A]₀⁻¹
    4. [A]₀½
24. The reaction 3A → B + C is zero-order when:
    1. Rate ∝ [A]²
    2. Rate is constant at any [A]
    3. Rate is unchanged by [B] or [C]
    4. Rate doubles if [B] doubles
25. Time for zero-order reaction to complete if [A]₀ is initial concentration and k₀ is rate constant:
    1. [A]₀ - [A]/2
    2. [A]₀/2k₀
    3. [A]₀ - [A]/2
    4. 2k₀/[A]₀
26. Rate equation for 2A + B → C if order is zero:
    1. Rate = k[A]²[B]
    2. Rate = k[A][B]⁰
    3. Rate = k[A]⁰[B]⁰
    4. Rate = k[A]⁰[B]²
27. First-order reaction has t₁/₂ = 32 s. If [A] = 0.062 M after 2.0 min, find [A]₀.
    1. 0.834 M
    2. 0.069 M
    3. 0.091 M
    4. 0.075 M
28. Time for [A] to reduce to half its initial value is called:
    1. Reaction life
    2. Half-life
    3. Half reactant
    4. Ideal life
29. Zero-order reaction: X → Product. If [X]₀ = 0.02 M, t₁/₂ = 10 min. For [X]₀ = 0.04 M, t₁/₂ is:
    1. 10 min
    2. 5 min
    3. 20 min
    4. Cannot be predicted
30. For a certain first order reaction, it is found that it takes 156 seconds for the concentration of reactant to fall from 0.100 M to 0.0500 M. How much time would it take for the concentration of reactant to fall from 0.0500 M to 0.0250 M?
    1. 165 seconds
    2. 76 seconds
    3. 156 seconds
    4. 98 seconds
31. The thermal decomposition of ammonium nitrite (NH₄NO₂) into molecular nitrogen and water is a first-order process: NH₄NO₂(g) → 2H₂O(g) + N₂(g). At 680°C the rate constant for this reaction is 0.0198 s⁻¹. How long will it take for an initial concentration of 0.57 M to fall to 0.013 M?
    1. 191 s
    2. 183 s
    3. 57 s
    4. 83 s
32. A catalyst functions by:
    1. lowering the energy of the reactants
    2. lowering the energy of the products
    3. decreasing the rate of the reverse reaction
    4. providing a reaction path with a lower activation energy
33. The rate of a reaction doubles when the concentration of one of the reactants is doubled. In terms of the concentration of this reactant, the reaction is:
    1. Zeroth-order
    2. First-order
    3. Second-order
    4. Third-order
34. Catalyzed reactions are faster because:
    1. The concentrations are higher
    2. The temperature is higher
    3. The mechanism is different
    4. None of the above
35. For a first-order reaction, a plot is made of ln([A]/[A]₀) versus time in seconds. The slope of the line is equal to……., and will have the units of…….:
    1. The rate constant, mol L⁻¹ s⁻¹
    2. The rate constant, s
    3. The rate constant, s⁻¹
    4. None of the above
36. A reaction involving two different reactants can never be a:
    1. First-order reaction
    2. Bimolecular reaction
    3. Second-order reaction
    4. Unimolecular reaction
37. The reaction: A → B follows first-order kinetics. The time taken for 0.8 mol of A to produce 0.6 mol of B is 1 hour. What is the time taken for conversion of 0.9 mol of A to produce 0.675 mol of B?
    1. 0.5 h
    2. 1 h
    3. 2 h
    4. 0.25 h
38. Which of the following statements is true about ORDER of a reaction?
    1. It is obtained from a single balanced equation
    2. It is always a whole number
    3. It cannot be obtained from a balanced chemical equation
    4. It reveals some basic facts about reaction mechanism
39. One of the following statements is not true about the MOLECULARITY of a reaction:
    1. It is always a whole number
    2. It is generally not exceeding three and never zero
    3. It reveals some basic facts about reaction mechanism
    4. It does not reveal anything about the mechanism of the reaction
40. The rate constant of a zero-order reaction is 0.2 (mol/L) hour⁻¹. What will be the initial concentration of the reactant if after an hour its concentration is 0.089 mol/L?
    1. 0.28 mol/L
    2. 0.15 mol/L
    3. 0.58 mol/L
    4. 0.189 mol/L
41. A first-order reaction is 60% complete at the end of 90 min, what is the value of rate constant in sec⁻¹?
    1. 1.69 × 10⁻⁴ s⁻¹
    2. 1.98 × 10⁻⁴ s⁻¹
    3. 1.75 × 10⁻⁴ s⁻¹
    4. 1.70 × 10⁻³ s⁻¹
42. The half-life for the first-order decomposition of N₂O₅ is 2.05 × 10⁴ s. How long will it take for a sample of this compound to decay to 80% of its initial value?
    1. 8.97 × 10⁷ s
    2. 1.51 × 10⁶ s
    3. 6.60 × 10⁷ s
    4. 1.48 × 10⁷ s
43. Which of the following statements is TRUE about a first-order reaction?
    1. Its half-life varies inversely as the initial concentration
    2. Its half-life does not depend on the initial concentration of the reactants
    3. Its rates does not depend on the nature of the reactants
    4. It is not a fast reaction
44. The optical rotations of sucrose in 0.5 M HCl measured in seconds at 0, 20 and ∞ are +32.4, +25.5 and -11.1 respectively. Determine the order of the reaction.
    1. 3
    2. 2
    3. 1
    4. 0
45. A first-order reaction is 40% complete at the end of 50 min, in how many minutes will the reaction be 80% complete?
    1. 169 min
    2. 198 min
    3. 158 min
    4. 178 min
46. The first-order decomposition of H₂O₂ was studied by titrating it at different time with KMnO₄. Calculate the velocity constant; if at t = 0, the volume of KMnO₄ used was 25 ml and at 30 minutes the volume of KMnO₄ used was 12.5 ml.
    1. 0.00228 s⁻¹
    2. 0.0232 s⁻¹
    3. 0.232 s⁻¹
    4. 0.222 s⁻¹
47. A reaction has the energy of activation of 84 kJ mol⁻¹, what is the effect on the rate (other things being equal) of raising the temperature from 100°C to 110°C (R = 8.314 J mol⁻¹ K⁻¹)?
    1. No effect on the rate of the reaction
    2. The rate of the reaction increases
    3. The rate of the reaction decreases
    4. The rate of the reaction is doubled
48. After 300 seconds, half of nitrogen (IV) oxide gas has reacted. How much of the gas will react after 900 second if the reaction is first-order?
    1. 66.7%
    2. 87.5%
    3. 99.9%
    4. 78.5%
49. The half-life of a radioactive element with mass number 234 g is 2.5 × 10⁵ years. How long after the isolation of a sample of this isotope will only one-sixth of the original mass be left?
    1. 4.87 × 10⁵ years
    2. 6.47 × 10⁵ years
    3. 8.47 × 10⁵ years
    4. 7.48 × 10⁵ years

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