NUMS — Medical & Dental Entry Test
Take the NUMS Chemistry — Kinetics, Acids and Bases, Electrochemistry (Set 4): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Initial-rate data for A + B → products: with [A] = 0.10 M and [B] = 0.10 M the rate is 2.0 × 10⁻³ M s⁻¹; doubling only [A] quadruples the rate, and doubling only [B] doubles it. What is the rate when [A] = 0.30 M and [B] = 0.20 M?
Answer: C. 0.036 M s⁻¹
Explanation: Rate = k[A]²[B]; k = 2.0 × 10⁻³/(0.10² × 0.10) = 2 M⁻² s⁻¹. Rate = 2 × 0.30² × 0.20 = 0.036 M s⁻¹.
Q2. A first-order reaction is 75% complete in 40 minutes. How long does it take to be 87.5% complete?
Answer: B. 60 minutes
Explanation: 75% complete = 2 half-lives, so t½ = 20 min. 87.5% complete = 3 half-lives (1 → ½ → ¼ → ⅛ remaining) = 60 min.
Q3. The decomposition of a gas on a hot platinum surface is zero order with k = 0.020 M min⁻¹. What is the half-life when the initial concentration is 0.50 M?
Answer: D. 12.5 min
Explanation: For zero order t½ = [A]₀/2k = 0.50/(2 × 0.020) = 12.5 min. It shrinks as the starting concentration falls.
Q4. A second-order reaction 2A → products has k = 0.50 M⁻¹ s⁻¹ (rate = k[A]²). What is the half-life when [A]₀ = 0.40 M?
Answer: C. 5.0 s
Explanation: t½ = 1/(k[A]₀) = 1/(0.50 × 0.40) = 5.0 s.
Q5. The rate constant of a reaction increases four-fold when the temperature is raised from 300 K to 320 K. What is the activation energy? (R = 8.314 J K⁻¹ mol⁻¹)
Answer: B. 55.3 kJ mol⁻¹
Explanation: ln 4 = (Ea/R)(1/300 − 1/320): Ea = 8.314 × 1.386/(2.083 × 10⁻⁴) = 55323 J = 55.3 kJ mol⁻¹.
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Q6. A catalyst lowers the activation energy of a reaction by 10 kJ mol⁻¹ without changing the pre-exponential factor. By roughly what factor does the rate constant increase at 300 K? (R = 8.314 J K⁻¹ mol⁻¹)
Q7. For a reaction of a single reactant, the half-life doubles whenever the initial concentration is halved. What is the order of the reaction?
Q8. A proposed mechanism for 2NO₂ + F₂ → 2NO₂F is: NO₂ + F₂ → NO₂F + F (slow); NO₂ + F → NO₂F (fast). What rate law does it predict?
Q9. What are the units of the rate constant for a third-order reaction when concentration is in mol L⁻¹ and time in seconds?
Q10. The rate law for H₂ + Br₂ → 2HBr at the start of the reaction, before much HBr has built up, is rate = k[H₂][Br₂]^½. What is the overall order at that stage?
Q11. What is the pH of a 1.0 × 10⁻³ M solution of Ba(OH)₂ at 25 °C, assuming complete dissociation?
Q12. 100 mL of 0.10 M HCl is mixed with 50 mL of 0.10 M NaOH. What is the pH of the resulting solution?
Q13. What is the pH of 0.10 M ammonia solution at 25 °C? (Kb = 1.8 × 10⁻⁵)
Q14. What is the pH of a buffer containing 0.20 M NH₃ and 0.10 M NH₄Cl at 25 °C? (Kb of NH₃ = 1.8 × 10⁻⁵)
Q15. 100 mL of a buffer is 0.10 M in acetic acid and 0.10 M in sodium acetate (pKa = 4.74). What is its pH after 10 mL of 0.10 M HCl is added?
Q16. What is the pH of 0.10 M NH₄Cl solution at 25 °C? (pKa of NH₄⁺ = 9.26)
Q17. At 60 °C the ionic product of water is Kw = 9.6 × 10⁻¹⁴. What is the pH of pure water at this temperature, and is it acidic, basic or neutral?
Q18. 25 mL of 0.10 M of a weak monoprotic acid is titrated with 0.10 M NaOH. What is true of the solution after 12.5 mL of NaOH has been added?
Q19. A strong acid is titrated against a weak base such as NH₃. Why is methyl orange (range 3.1–4.4) a better indicator than phenolphthalein (range 8.3–10)?
Q20. Which of these species is amphiprotic, able to act as both a Brønsted acid and a Brønsted base?
Q21. Chromium pentoxide, CrO₅, contains one oxide ion and two peroxide (O₂²⁻) groups. What is the oxidation number of chromium in it?
Q22. In acid solution MnO₄⁻ oxidises Fe²⁺ to Fe³⁺ (MnO₄⁻ → Mn²⁺). What volume of 0.10 M FeSO₄ is oxidised by 20.0 mL of 0.020 M KMnO₄?
Q23. The Daniell cell has E°cell = 1.10 V with 2 electrons transferred. What is ΔG° for the cell reaction? (F = 96 485 C mol⁻¹)
Q24. Using log K = nE°/0.0592 at 25 °C, what is the equilibrium constant for Zn + Cu²⁺ ⇌ Zn²⁺ + Cu, for which E°cell = 1.10 V?
Q25. A current of 19 297 C is passed through acidified water between inert electrodes (1 F = 96 485 C). What total volume of gas, H₂ plus O₂, is collected at STP? (22.4 L mol⁻¹)
Q26. Aqueous silver nitrate is electrolysed between platinum electrodes. What happens at the anode, and how does the solution change?
Q27. What is the electrode potential of a hydrogen electrode (H₂ at 1 atm) dipped in a solution of pH 3 at 25 °C? (E° = 0 V)
Q28. A 0.10 M solution of a weak acid has molar conductivity 39 S cm² mol⁻¹, while its limiting value is 390 S cm² mol⁻¹. What is Ka? (α = Λ/Λ°)
Q29. In an alkaline hydrogen–oxygen fuel cell, what reaction occurs at the anode?
Q30. A galvanised (zinc-coated) iron sheet and a tin-plated iron sheet are both scratched, exposing the iron. (E° Zn²⁺/Zn = −0.76 V, Fe²⁺/Fe = −0.44 V, Sn²⁺/Sn = −0.14 V.) What happens to the iron?