NTS NAT-IM (Medical / Pre-Medical Track) Chemistry Chemical Equilibrium — Set 3

Chemical Equilibrium MCQs set 3 for NTS NAT-IM (Medical / Pre-Medical Track) Chemistry — 20 solved questions.

NTS NAT-IM (Medical / Pre-Medical Track) Chemistry Chemical Equilibrium — Set 3

  1. Question 1

    Q1. In a closed vessel, a reversible chemical reaction reaches a state where macroscopic properties like color and pressure remain unchanged. What characterizes this state?

    • A) The concentrations of reactants and products become exactly equal
    • B) The rates of forward and reverse reactions become equal
    • C) The reaction stops completely and no further change occurs
    • D) The concentration of products becomes much higher than reactants

    Answer: The rates of forward and reverse reactions become equal

    Explanation: Dynamic equilibrium occurs when forward and reverse rates are equal. Option A is wrong because concentrations become constant, not necessarily equal.

  2. Question 2

    Q2. Hydrogen gas reacts with iodine vapors to form hydrogen iodide in a reversible process. What are the units for the equilibrium constant Kc for this reaction?

    • A) mol dm-3
    • B) mol-1 dm3
    • C) mol-2 dm6
    • D) No units

    Answer: No units

    Explanation: Kc is unitless when moles of reactants equal moles of products. Option A fails because it assumes units always exist for Kc.

  3. Question 3

    Q3. A mixture of nitrogen and hydrogen is at equilibrium forming ammonia. If helium gas is added at constant volume, what happens to the equilibrium position?

    • A) The equilibrium shifts towards the product side
    • B) The equilibrium shifts towards the reactant side
    • C) The equilibrium position remains unchanged
    • D) The value of Kc increases significantly

    Answer: The equilibrium position remains unchanged

    Explanation: Adding inert gas at constant volume does not change partial pressures or concentrations. Option D is wrong because pressure increases without shifting equilibrium.

  4. Question 4

    Q4. In the industrial Haber process, which combination of conditions is strategically used to maximize the percentage yield of ammonia?

    • A) High pressure and continuous removal of ammonia
    • B) High temperature and low pressure
    • C) Low pressure and addition of a catalyst
    • D) High temperature and addition of an inhibitor

    Answer: High pressure and continuous removal of ammonia

    Explanation: High pressure favors the side with fewer moles (ammonia). Option B is wrong because high temperature shifts exothermic reactions backward, reducing yield.

  5. Question 5

    Q5. Phosphorus pentachloride dissociates into PCl3 and Cl2 at 250 degrees Celsius. Comparing the numerical values of Kp and Kc, which statement is true?

    • A) Kp = Kc
    • B) Kp > Kc
    • C) Kp < Kc
    • D) Kp = Kc / RT

    Answer: Kp > Kc

    Explanation: Kp = Kc(RT)^delta_n; for PCl5 dissociation, delta_n is +1. Option A is wrong because it ignores the RT factor for gas moles.

  6. Question 6

    Q6. The ionic product of water, Kw, is measured at 40 degrees Celsius. How does this value compare to the standard value at 25 degrees Celsius?

    • A) Exactly 1.0 x 10^-14
    • B) Less than 1.0 x 10^-14
    • C) Greater than 1.0 x 10^-14
    • D) Exactly 7.0 x 10^-7

    Answer: Greater than 1.0 x 10^-14

    Explanation: Water dissociation is endothermic, so Kw increases with temperature. Option A is wrong because 1.0 x 10^-14 is only at 25 degrees.

  7. Question 7

    Q7. During qualitative salt analysis, dilute HCl is added before passing H2S gas to precipitate Group II radicals. What is the chemical purpose of HCl?

    • A) To increase the solubility of Group II sulphides
    • B) To increase the ionization of H2S gas
    • C) To provide a basic medium for precipitation
    • D) To suppress H2S ionization via common ion effect

    Answer: To suppress H2S ionization via common ion effect

    Explanation: HCl provides a common ion (H+) that suppresses H2S ionization, allowing only Group II to precipitate. Option C fails because basicity is irrelevant.

  8. Question 8

    Q8. A chemist calculates the reaction quotient Qc for a system and finds it is less than the equilibrium constant Kc. What is the predicted direction?

    • A) The reaction proceeds in the backward direction
    • B) The reaction proceeds in the forward direction
    • C) The reaction is already at equilibrium
    • D) The reaction stops and Kc becomes zero

    Answer: The reaction proceeds in the forward direction

    Explanation: If Qc < Kc, the reaction proceeds forward to reach equilibrium. Option A is wrong because it describes the reverse shift.

  9. Question 9

    Q9. In the synthesis of sulfur trioxide via the Contact process, finely divided platinum is used. What effect does this catalyst have?

    • A) It increases the rate of both forward and backward reactions
    • B) It shifts the equilibrium to favor product formation
    • C) It increases the value of the equilibrium constant Kc
    • D) It decreases the rate of the backward reaction only

    Answer: It increases the rate of both forward and backward reactions

    Explanation: Catalysts lower activation energy for both directions equally. Option B is wrong because catalysts do not shift the equilibrium position.

  10. Question 10

    Q10. A laboratory technician needs to prepare a buffer solution to maintain a pH of approximately 4.7. Which pair of chemicals should be selected?

    • A) HCl and NaCl
    • B) NaOH and NaCH3COO
    • C) CH3COOH and CH3COONa
    • D) NH4OH and NH4Cl

    Answer: CH3COOH and CH3COONa

    Explanation: An acidic buffer consists of a weak acid and its salt with a strong base. Option A is a strong acid/salt mix.

  11. Question 11

    Q11. If the molar solubility of Lead (II) chloride (PbCl2) is represented by 's', what is the correct expression for its Solubility Product (Ksp)?

    • A) s^2
    • B) 2s^2
    • C) s^3
    • D) 4s^3

    Answer: 4s^3

    Explanation: Ksp = [Pb][Cl]^2 = (s)(2s)^2 = 4s^3. Option C is wrong because it forgets to square the chloride concentration and the coefficient.

  12. Question 12

    Q12. Which two scientists are credited with formulating the Law of Mass Action, relating the rate of reaction to active masses?

    • A) Le Chatelier and Braun
    • B) Guldberg and Waage
    • C) Henderson and Hasselbalch
    • D) Sorensen and Lewis

    Answer: Guldberg and Waage

    Explanation: Guldberg and Waage proposed the Law of Mass Action in 1864. Option A refers to the principle of equilibrium shifts.

  13. Question 13

    Q13. When sodium acetate (CH3COONa) is dissolved in water, the resulting solution is tested with litmus. What is the expected nature of the solution?

    • A) Basic with pH > 7
    • B) Acidic with pH < 7
    • C) Neutral with pH = 7
    • D) Highly acidic with pH < 2

    Answer: Basic with pH > 7

    Explanation: Sodium acetate is a salt of a weak acid and strong base, producing OH- ions. Option B is wrong as it's acidic.

  14. Question 14

    Q14. For a multi-step reversible reaction, how is the equilibrium constant Kc related to the rate constants of the forward and reverse elementary steps?

    • A) The sum of the rates of all elementary steps
    • B) The product of the rate constants of all steps
    • C) The ratio of the forward and backward rate constants
    • D) The rate of the slowest step only

    Answer: The ratio of the forward and backward rate constants

    Explanation: Kc is the ratio of rate constants of elementary steps (kf/kr). Option D is wrong because Kc is independent of the path.

  15. Question 15

    Q15. Consider the gas phase equilibrium: N2O4 (colorless) <=> 2NO2 (brown). What happens to the color intensity if the pressure of the system is increased?

    • A) The reaction shifts towards the right (NO2 side)
    • B) The reaction shifts towards the left (N2O4 side)
    • C) The concentration of NO2 remains unchanged
    • D) The value of Kc decreases significantly

    Answer: The reaction shifts towards the left (N2O4 side)

    Explanation: Increasing pressure shifts equilibrium to the side with fewer moles (N2O4). Option A is wrong because it favors the forward direction.

  16. Question 16

    Q16. A reaction has an equilibrium constant Kc of 10^15. What can be inferred about the extent of this reaction at equilibrium?

    • A) The reaction hardly proceeds in the forward direction
    • B) The reactants and products are in equal concentrations
    • C) The reaction is at a very early stage of progress
    • D) The reaction is almost complete at equilibrium

    Answer: The reaction is almost complete at equilibrium

    Explanation: A large Kc (>10^3) indicates the reaction has almost gone to completion. Option A is for very small Kc.

  17. Question 17

    Q17. In an acidic buffer, the concentration of the weak acid and its conjugate salt are made equal. What is the resulting pH?

    • A) The pH will always be 7.0
    • B) The pH will be equal to the pKb of the base
    • C) The pH will be equal to the pKa of the acid
    • D) The pH will be independent of the pKa

    Answer: The pH will be equal to the pKa of the acid

    Explanation: pH = pKa + log([Salt]/[Acid]); if concentrations are equal, pH = pKa. Option A is wrong because pH isn't always 7.

  18. Question 18

    Q18. Under what condition will a precipitate form when two solutions containing ions of a sparingly soluble salt are mixed?

    • A) The ionic product exceeds the solubility product
    • B) The solubility product exceeds the ionic product
    • C) The solution is exactly saturated
    • D) The temperature of the solution is decreased

    Answer: The ionic product exceeds the solubility product

    Explanation: Ionic product (Qi) > Ksp results in precipitation. Option B is wrong because Qi < Ksp means the solution is unsaturated.

  19. Question 19

    Q19. Comparing two acids, Acid X has a Ka of 1.8 x 10^-5 and Acid Y has a Ka of 4.5 x 10^-4. Which statement is correct?

    • A) The acid is weaker and has a higher pKa
    • B) The acid is stronger and has a lower pKa
    • C) The acid is stronger and has a higher pKa
    • D) The acid is weaker and has a lower pKa

    Answer: The acid is stronger and has a lower pKa

    Explanation: Stronger acids have larger Ka and smaller pKa values. Option A is wrong because Ka and acidity are directly proportional.

  20. Question 20

    Q20. In Group III qualitative analysis, NH4Cl is added before NH4OH. What is the role of NH4Cl in this specific procedure?

    • A) To increase the concentration of OH- ions
    • B) To make the solution more acidic
    • C) To decrease the concentration of OH- ions
    • D) To react with Group III metal ions directly

    Answer: To decrease the concentration of OH- ions

    Explanation: NH4Cl provides a common ion (NH4+) that reduces NH4OH ionization, preventing Group IV precipitation. Option D is wrong.

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Level 1

In a closed vessel, a reversible chemical reaction reaches a state where macroscopic properties like color and pressure remain unchanged. What characterizes this state?