Question

Which is/are correct statements :

(1) When 100 ml of 0.1 M NaCN solution is titrated with 0.1 M HCl solution the variation of pH of solution with volume of HCl added will be (as shown in figure):

(2) Variation of degree of dissociation a with concentration for a weak electrolyte at a particular temperature is best represented by (as shown in figure):

(3) 0.1 M acetic acid solution is titrated against 0.1 M NaOH solution. The difference in pH between 1/4 and 3/4 stages of neutralization of acid will be 2 log 3.

• A. a& c
• B. b & c
• C. a, b & c
• D. b only

Answer 1

Answer: (C)

a, b & c

Answer 2

(1) Initially pH will decrease fast, then slowly due to buffer formation and then will decrease fast as buffer action diminishes.

(2) For a weak electrolyte

Ka = $\frac{C\alpha^{2}}{(1 - \alpha)}$

when a << 1 then a = $\sqrt{\frac{K_{a}}{C}}$as C increases

$\Rightarrow$ a decreases

as C is tending to zero $\Rightarrow$ a will be unity

(3) At 1/4th neutralization

CH3COOH + NaOH $\longrightarrow$ CH3COONa + H2O

$\left( 0.1 \times \frac { 3 } { 4 } \right)$ $\left( 0.1 \times \frac { 1 } { 4 } \right)$

pH = pKa + log $\frac{\lbrack CH_{3}COO^{-}\rbrack}{\lbrack CH_{3}COOH\rbrack}$= pKa + log $\left( \frac{1}{3} \right)$

At 3/4th neutralization

pH = pKa + log 3

so difference in pH = D(pH) = log 3 – log = 2 log 3