JEE Chemistry Equilibrium MCQs Set 11

Answer: (a) \( 1\times10^{-8} \)

Explanation:
1. α = 0.01% = \( 10^{-4} \).
2. For a weak acid \( K_a=C\alpha^2=1\times(10^{-4})^2 \) = \( 1\times10^{-8} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) \( 2.1\times10^{-4} \)

Explanation:
1. \( K_a=C\alpha^2=0.2\times(0.032)^2 \) = \( 2.05\times10^{-4} \), i.e. \( 2.1\times10^{-4} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) \( 10^{-9} \)

Explanation:
1. α = \( 10^{-4} \).
2. \( K_a=0.1\times(10^{-4})^2=10^{-9} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (c) \( 6.3\times10^{-6} \) M

Explanation:
1. [H+] = [CN-] = \( \sqrt{K_aC}=\sqrt{4\times10^{-10}\times0.1} \).
2. = \( \sqrt{4\times10^{-11}}=6.3\times10^{-6} \) M.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) \( 10^{-2} \)

Explanation:
1. \( \alpha=\sqrt{K_a/C}=\sqrt{10^{-5}/0.1}=\sqrt{10^{-4}} \) = \( 10^{-2} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) 49

Explanation:
1. \( BaCl_2 \) gives 3 ions, so i = 1 + 2α.
2. 1.98 = 1 + 2α, so α = 0.49 = 49%.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) \( \ce{CH3COO-} \)

Explanation:
1. The weaker the conjugate acid, the stronger the conjugate base.
2. Conjugate acids: HCl (strong), \( \ce{CH3COOH} \) (pKa 4.8), \( \ce{HSO4-} \) (pKa 2) and \( \ce{HNO2} \) (pKa 3.3).
3. Acetic acid is the weakest, so acetate is the strongest base.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (c) \( \ce{NH3} \)

Explanation:
1. Proton affinity is the tendency to accept a proton through a lone pair.
2. Nitrogen is small and its lone pair is concentrated, so \( \ce{NH3} \) accepts a proton most readily here.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) \( \ce{BI3} \)

Explanation:
1. Boron trihalides accept an electron pair into the empty p-orbital.
2. Back-donation of halogen lone pairs is strongest for F and weakest for I, so \( \ce{BI3} \) has the most electron-deficient boron and the greatest Lewis acidity.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) CO

Explanation:
1. A Lewis acid accepts an electron pair.
2. SiCl4 (empty d-orbitals), SO3 (electron-deficient S) and Zn2+ (empty orbitals) all accept pairs.
3. CO is a lone-pair donor (a Lewis base).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) \( \ce{NH3} \)

Explanation:
1. The conjugate acid is formed by adding a proton: \( \ce{NH2- + H+ -> NH3} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) an acid

Explanation:
1. \( \ce{NH3 + H2O <=> NH4+ + OH-} \): water donates a proton to ammonia, so it behaves as a Bronsted acid.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) hydrolysis of \( \ce{Fe^3+} \)

Explanation:
1. \( \ce{Fe^3+} \) is a small, highly charged cation that hydrolyses: \( \ce{Fe^3+ + 3H2O <=> Fe(OH)3 + 3H+} \).
2. The H+ released makes the solution acidic.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (c) \( \ce{Na2CO3} \)

Explanation:
1. \( \ce{Na2CO3} \) is the salt of a strong base and a weak acid: \( \ce{CO3^2- + H2O <=> HCO3- + OH-} \).
2. The solution is alkaline. \( \ce{NaNO3} \) is neutral, \( \ce{NH4Cl} \) acidic, and \( \ce{Fe2O3} \) is insoluble.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) \( \ce{HCO3-} \)

Explanation:
1. \( \ce{HCO3-} \) can donate a proton (giving \( \ce{CO3^2-} \)) or accept one (giving \( \ce{H2CO3} \)).
2. Such species are amphiprotic.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) an electron pair acceptor

Explanation:
1. Lewis acids accept an electron pair; Lewis bases donate one.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (c) basic

Explanation:
1. NaCN is the salt of a strong base (NaOH) and a weak acid (HCN).
2. The cyanide ion hydrolyses: \( \ce{CN- + H2O <=> HCN + OH-} \), giving a basic solution.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) \( \ce{Ba(NO3)2} \)

Explanation:
1. \( \ce{Ba(NO3)2} \) is the salt of a strong base and a strong acid, so neither ion hydrolyses.
2. The others contain hydrolysing cations and give acidic solutions.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) \( \ce{H2SO4} \)

Explanation:
1. Sulphuric acid is a strong acid (first ionisation essentially complete).
2. The other three are weak acids.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) 100%

Explanation:
1. As concentration tends to zero, the degree of ionisation (Ostwald's law) tends to 1 for every electrolyte.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) None of these

Explanation:
1. \( \ce{PbI2 <=> Pb^2+ + 2I-} \): \( K_{sp}=4S^3 \).
2. \( 4\times(0.005)^3=5\times10^{-7} \), which matches none of the first three values.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (d) \( 1.0\times10^{-11} \)

Explanation:
1. α = 0.001% = \( 10^{-5} \).
2. \( K_a=C\alpha^2=0.1\times10^{-10}=1.0\times10^{-11} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (a) 1.0

Explanation:
1. A smaller pKa means a larger Ka and a stronger acid.
2. The strongest acid has pKa = 1.0.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (c) \( 10^{-12} \)

Explanation:
1. In pure water [H3O+] = [OH-] = \( 10^{-6} \).
2. \( K_w=10^{-6}\times10^{-6}=10^{-12} \).

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.

Answer: (b) 7

Explanation:
1. Equal milliequivalents of strong acid and strong base (2 meq each) neutralise completely.
2. The solution is that of KCl, a neutral salt: pH = 7.

Teacher's Note:
1. Decide first whether the species is an acid, a base or a salt and which constant (Ka, Kb, Kh, Ksp) applies.
2. Check the final pH or concentration for reasonableness: acids give pH below 7, bases above 7.