Common ion effect MCQ - Practice Questions & Answers

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Quick Facts

pH of weak acid + strong acid is considered one the most difficult concept.
17 Questions around this concept.

Solve by difficulty

EASY

Which one of the following statements is not true?

The conjugate base of $H_{2}PO{_{4}}^{-}$ is $HPO{_{4}}^{2-}$

$pH+pOH= 14\: for \: all \: aqueous\: solutions.$

$The\: pH\: of\: 1\times 10^{-8}M\: H\! C\! L \: is \: 8$

96,500 coulombs of electricity when passed through a $CuSO_4$ solution deposits 1 gram equivalent of copper at the cathode.

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Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R:

Assertion A: pKa value of phenol is 10.0 while that of ethanol is 15.9

Reason R : Ethanol is stronger acid than phenol.

In the light of the above statements, choose the correct answer from the options given below:

A is true but R is false.

Both A and R are true but R is NOT the correct explanation of A.

Both A and R are true and R is the correct explanation of A.

A is false bur R is true.

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Concepts Covered - 2

Common ion effect

The value of the degree of dissociation for a weak electrolyte is decreased by the addition of a strong electrolyte having a common ion. As a result of this effect, the concentration of the uncommon ion of the weak electrolyte decreases.
For example:

$\mathrm{NH_{4}OH\: \rightleftharpoons \: NH_{4}^{+}\: +\: OH^{-}}$
weak electrolyte

$\mathrm{NH_{4}Cl\: \rightleftharpoons \: NH_{4}^{+}\: +\: Cl^{-}}$
Common ion
Here $\alpha$ for NH₄OH will be decreased by NH₄Cl
$\mathrm{CH_{3}COOH\: \rightleftharpoons \: CH_{3}COO^{-}\: +\: H^{+}}$
$\mathrm{CH_{3}COONa\: \rightleftharpoons \: CH_{3}COO^{-}\: +\: Na^{+}}$
Here $\alpha$ for CH₃COOH will be decreased by CH₃COONa

Applications of Common ion effect

The solubility of a partially soluble salt decreases due to the common ion effect. For example, the presence of AgNO₃or KCl decreases the solubility of AgCl in water.
Salting out of soap by addition of NaCl.
Purification of NaCl by passing HCl gas.

Isohydric Solution: These are the solutions having the same concentration of common ions.

pH of weak acid + strong acid

The pH of a mixture of a weak acid and a strong acid can be understood using the following example.

Weak acid: H₂S (0.1M)
Strong acid: HCl (0.3M)
The K_a value for this mixture = 1.2 x 10^-20

The chemical equation for H₂S is given as follows:

$\mathrm{H_{2}S\: \rightleftharpoons \: 2H^{+}\: +\: S^{2-}}$
Initial: c 0 0
Equil: c - c? c? c?

Thus, the equilibrium constant K_a is given as follows:

$\\\mathrm{K_{a}\: =\: \frac{[c\alpha ]^{2}.c\alpha }{c(1-\alpha )}\: =\: \frac{[H^{+}]^{2}.c\alpha }{c(1-\alpha )}}\\\\\mathrm{1.2\, \,x\, 10^{-20}\: = \frac{[c\alpha \: +\: 0.3].c\alpha }{c(1-\alpha )}\: =\:(0.3)^{2}\: =\: 0.09\alpha}\\\\\mathrm{\mathbf{Thus},\: \alpha \: =\: \frac{1.2\: x\: 10^{-20}}{0.09}\: =\: 13.3\: x\: 10^{-20}}$

Again, the chemical equation for HCl is given as follows:
$\mathrm{HCl\: \rightarrow \: H^{+}\: +\: Cl^{-}}$
Initial: 0.3 0 0
Equil: 0 0.3 0.3

Thus, total [H⁺] for the mixture = c? + 0.3
[H⁺] = 3 x 10^-1 (? is very small for weak acids)

Now, the pH for the mixture is given as follows:
$\\\mathrm{pH\: =\: -log[H^{+}]\: =\: -log[3\: x\: 10^{-1}]\: =\: -log\, 3\: +\: 1\: =\: -0.47\: +\: 1}\\\\\mathrm{pH\: =\: 0.53}$