Chemistry Objective Questions and Answers: Ionic Equilibrium
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Multiple Choice Questions
(a) \( \ce{NH4OH + NaOH} \)
(b) \( \ce{HCOOH + CH3COONa} \)
(c) 40 mL of 0.1 M NaCN + 20 mL of 0.1 M HCl
(d) none of these
Show Answer & Explanation
Answer: (c) 40 mL of 0.1 M NaCN + 20 mL of 0.1 M HCl
Explanation:
1. NaCN \( =4 \) mmol and HCl \( =2 \) mmol react to give 2 mmol HCN and leave 2 mmol \( \ce{CN-} \).
2. A weak acid with its conjugate base in comparable amounts is a buffer.
3. The first mixture has a strong base; the second has a weak acid with a salt of a different acid.
Teacher's Note:
1. A buffer needs a weak acid and its own conjugate base (or the reverse).
(a) [conjugate acid] : [base] = 1 : 10
(b) [conjugate acid] = [base]
(c) [conjugate acid] : [base] = 10 : 1
(d) none of these
Show Answer & Explanation
Answer: (c) [conjugate acid] : [base] = 10 : 1
Explanation:
1. \( pOH=pK_b+\log\frac{[\text{conjugate acid}]}{[\text{base}]} \).
2. For the difference to be 1, the ratio must be 10 : 1.
Teacher's Note:
1. Apply Henderson's equation for a basic buffer.
(a) 3
(b) 1
(c) 2
(d) 0
Show Answer & Explanation
Answer: (a) 3
Explanation:
1. The three pairs are \( \ce{H3PO4/H2PO4-} \), \( \ce{H2PO4-/HPO4^2-} \) and \( \ce{HPO4^2-/PO4^3-} \).
Teacher's Note:
1. One buffer per ionisation step.
(a) [acid] = [conjugate base]
(b) [acid] \( \times10 \) = [conjugate base]
(c) [acid] = [conjugate base] \( \times10 \)
(d) none of these
Show Answer & Explanation
Answer: (b) [acid] \( \times10 \) = [conjugate base]
Explanation:
1. \( pH-pK_a=\log\frac{[\text{base}]}{[\text{acid}]}=1 \).
2. So [conjugate base] = 10 [acid].
Teacher's Note:
1. pH above pKa means more of the conjugate base.
(a) 0.4
(b) 0.05
(c) -0.05
(d) 2.5
Show Answer & Explanation
Answer: (a) 0.4
Explanation:
1. Buffer capacity \( =\frac{\text{moles of base added per litre}}{\Delta pH}=\frac{0.02}{0.05}=0.4 \).
Teacher's Note:
1. Capacity is the amount needed to change pH by one unit.
(a) \( 5+\log\frac13,\ 5,\ 5+\log3 \)
(b) \( 5+\log3,\ 4,\ 5+\log\frac13 \)
(c) \( 5-\log\frac13,\ 5,\ 5-\log3 \)
(d) \( 5-\log3,\ 4,\ 5+\log\frac13 \)
Show Answer & Explanation
Answer: (a) \( 5+\log\frac13,\ 5,\ 5+\log3 \)
Explanation:
1. \( pK_a=5 \); \( pH=5+\log\frac{\text{salt}}{\text{acid}} \).
2. At 25%: ratio 1/3. At 50%: ratio 1. At 75%: ratio 3.
Teacher's Note:
1. The salt to acid ratio is x : (100 - x).
(a) Only a
(b) a and b
(c) a and c
(d) b and c
Show Answer & Explanation
Answer: (a) Only a
Explanation:
1. Water has lone pairs, can donate a proton and can accept one, so it fits all three roles.
2. \( \ce{NH4+} \) has no lone pair and is only an acid; \( \ce{N^3-} \) is only a base.
Teacher's Note:
1. Amphiprotic species do both.
(a) \( \ce{[Al(H2O)6]^3+} \)
(b) \( \ce{[Fe(H2O)6]^3+} \)
(c) \( \ce{HPO4^2-} \)
(d) \( \ce{ClO3-} \)
Show Answer & Explanation
Answer: (d) \( \ce{ClO3-} \)
Explanation:
1. The hexa-aqua ions of Al and Fe release protons from coordinated water.
2. \( \ce{HPO4^2-} \) can donate a proton.
3. \( \ce{ClO3-} \) has no ionisable hydrogen.
Teacher's Note:
1. No hydrogen means no Bronsted acidity.
(a) \( \ce{H2O + CH3COOH \lt => H3O+ + CH3COO-} \)
(b) \( \ce{2NH3 + H2SO4 \lt => 2NH4+ + SO4^2-} \)
(c) \( \ce{NH3 + CH3COOH \lt => NH4+ + CH3COO-} \)
(d) \( \ce{Cu^2+ + 4NH3 \lt => [Cu(NH3)4]^2+} \)
Show Answer & Explanation
Answer: (d) \( \ce{Cu^2+ + 4NH3 \lt => [Cu(NH3)4]^2+} \)
Explanation:
1. The first three involve proton transfer.
2. In the last, \( \ce{NH3} \) donates a lone pair to \( \ce{Cu^2+} \) without any proton transfer.
Teacher's Note:
1. Look for proton transfer versus electron-pair donation.
(a) \( \ce{SnCl2} \) and \( \ce{SnCl4} \) only
(b) \( \ce{BF3} \) and \( \ce{SnCl2} \) only
(c) \( \ce{BF3} \) only
(d) \( \ce{BF3} \), \( \ce{SnCl2} \) and \( \ce{SnCl4} \)
Show Answer & Explanation
Answer: (d) \( \ce{BF3} \), \( \ce{SnCl2} \) and \( \ce{SnCl4} \)
Explanation:
1. \( \ce{BF3} \) has an incomplete octet.
2. \( \ce{SnCl2} \) has a vacant orbital and \( \ce{SnCl4} \) can expand its octet using d orbitals.
3. All three accept electron pairs.
Teacher's Note:
1. Electron-pair acceptors are Lewis acids.
(a) a stronger Lewis acid
(b) a stronger Lewis base
(c) a weaker Lewis acid
(d) a weaker Lewis base
Show Answer & Explanation
Answer: (c) a weaker Lewis acid
Explanation:
1. Lewis acidity increases with charge density.
2. \( \ce{Al^3+} \) has higher charge and smaller size, so \( \ce{Mg^2+} \) is the weaker Lewis acid.
Teacher's Note:
1. Compare charge to size ratios.
(a) \( \ce{HC2O4-} \) and \( \ce{PO4^3-} \)
(b) \( \ce{HPO4^2-} \) and \( \ce{C2O4^2-} \)
(c) \( \ce{PO4^3-} \) and \( \ce{C2O4^2-} \)
(d) \( \ce{HC2O4-} \) and \( \ce{HPO4^2-} \)
Show Answer & Explanation
Answer: (c) \( \ce{PO4^3-} \) and \( \ce{C2O4^2-} \)
Explanation:
1. In the forward direction \( \ce{PO4^3-} \) accepts a proton, so it is a base.
2. In the reverse direction \( \ce{C2O4^2-} \) accepts a proton, so it is the other base.
Teacher's Note:
1. Bases are proton acceptors on either side.
(a) \( \ce{NH3} \)
(b) \( \ce{C2H5OH} \)
(c) \( \ce{H2O} \)
(d) \( \ce{C6H6} \)
Show Answer & Explanation
Answer: (d) \( \ce{C6H6} \)
Explanation:
1. HCl needs a proton acceptor to show acidity.
2. Ammonia, ethanol and water accept protons; benzene does not.
Teacher's Note:
1. No base, no acid behaviour.
(a) \( \ce{NH3 + HCl -> NH4Cl} \)
(b) \( \ce{NH3 + H+ -> NH4+} \)
(c) \( \ce{NH3 + Na -> NaNH2 + 1/2 H2} \)
(d) \( \ce{NH3} \) cannot act as an acid
Show Answer & Explanation
Answer: (c) \( \ce{NH3 + Na -> NaNH2 + 1/2 H2} \)
Explanation:
1. In the third reaction \( \ce{NH3} \) loses \( \ce{H+} \) (to form \( \ce{NH2-} \)).
2. In the first two it accepts a proton.
Teacher's Note:
1. An acid donates a proton.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iii)
Show Answer & Explanation
Answer: (c) (iii) and (iv)
Explanation:
1. In (iii) \( \ce{NH3} \) takes a proton from water, so water acts as an acid.
2. In (iv) water takes a proton from HCl, so it acts as a base.
3. Both roles appear in (iii) and (iv).
Teacher's Note:
1. Amphoteric means acid in one reaction and base in another.
(a) weak conjugate base
(b) strong conjugate base
(c) weak conjugate acid
(d) strong conjugate acid
Show Answer & Explanation
Answer: (a) weak conjugate base
Explanation:
1. Acetic acid is a weak acid, so its conjugate base is a weak base.
Teacher's Note:
1. The weaker the acid, the stronger its conjugate base.
(a) \( \ce{ClO4-} \)
(b) \( \ce{HCO3-} \)
(c) \( \ce{F-} \)
(d) \( \ce{HSO4-} \)
Show Answer & Explanation
Answer: (b) \( \ce{HCO3-} \)
Explanation:
1. The conjugate acids are \( \ce{HClO4} \), \( \ce{H2CO3} \), HF and \( \ce{H2SO4} \).
2. \( \ce{H2CO3} \) is by far the weakest acid, so \( \ce{HCO3-} \) is the strongest base.
Teacher's Note:
1. Weakest conjugate acid gives the strongest conjugate base.
(a) \( \ce{Na2CO3} \)
(b) \( \ce{O^2-} \)
(c) \( \ce{CO3^2-} \)
(d) \( \ce{NH3} \)
Show Answer & Explanation
Answer: (d) \( \ce{NH3} \)
Explanation:
1. \( \ce{NH3} \) accepts a proton to form \( \ce{NH4+} \) and can donate one to give \( \ce{NH2-} \).
2. \( \ce{O^2-} \) and \( \ce{CO3^2-} \) cannot donate a proton.
Teacher's Note:
1. An amphiprotic species has both abilities.
(a) Arrhenius theory can explain the acidic or basic nature of substances in solvents other than water
(b) Arrhenius theory does not explain the acidic nature of \( \ce{AlCl3} \)
(c) The aqueous solution of \( \ce{Na2CO3} \) is alkaline although it contains no \( \ce{OH-} \) ions initially
(d) Aqueous \( \ce{CO2} \) is acidic although it contains no \( \ce{H+} \) ions initially
Show Answer & Explanation
Answer: (a) Arrhenius theory can explain the acidic or basic nature of substances in solvents other than water
Explanation:
1. Arrhenius theory is limited to aqueous solutions, so the first statement is wrong.
2. The other three are true and need Lewis or Bronsted ideas.
Teacher's Note:
1. Arrhenius theory is water-based only.
Assertion–Reason Questions
Assertion (A): When a small amount of acid or base is added to pure water, its pH changes.
Reason (R): Addition of an acid or base increases the degree of ionisation of water.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. The assertion is true: pure water has no buffering, so pH shifts.
2. The reason is false: added \( \ce{H+} \) or \( \ce{OH-} \) suppresses the ionisation of water (common ion effect).
Teacher's Note:
1. Water ionises less in acid or base.
Assertion (A): The aqueous solution of \( \ce{Na3PO4} \) is alkaline.
Reason (R): \( \ce{Na3PO4} \) in its aqueous solution undergoes anionic hydrolysis.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (a) Both A and R are true, and R is the correct explanation of A.
Explanation:
1. \( \ce{PO4^3-} \) is the anion of a weak acid and takes protons from water, giving \( \ce{OH-} \).
2. So the reason explains the assertion.
Teacher's Note:
1. Anionic hydrolysis makes a solution basic.
Assertion (A): To precipitate the fourth-group cations in qualitative analysis, the medium is made alkaline before passing \( \ce{H2S} \).
Reason (R): This is done to suppress the ionisation of \( \ce{H2S} \).
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. The assertion is true: group IV sulphides need a higher \( [\ce{S^2-}] \).
2. The reason is false: the alkaline medium increases ionisation of \( \ce{H2S} \) by removing \( \ce{H+} \).
Teacher's Note:
1. Alkali raises sulphide ion concentration.
Assertion (A): When an acid or a base is added to water at constant temperature, the pH changes.
Reason (R): This is due to a change in the ionic product of water.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. The assertion is true.
2. The reason is false: \( K_w \) depends only on temperature and stays constant; the pH changes because \( [\ce{H+}] \) and \( [\ce{OH-}] \) change.
Teacher's Note:
1. Kw changes only with temperature.
Assertion (A): The solubility of the salt of a strong base and weak acid changes when the pH of the solution is changed.
Reason (R): The solubility product \( (K_{sp}) \) depends on pH.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (c) A is true, but R is false.
Explanation:
1. The assertion is true: the anion is protonated at lower pH, so more salt dissolves.
2. The reason is false: \( K_{sp} \) is a constant at a given temperature.
Teacher's Note:
1. Ksp depends only on temperature.
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Multiple Choice Questions (MCQs) for JEE Chemistry Ionic Equilibrium
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