Question

An acid-base indicator which is a weak acid has a pKIn value = 5.45. At what cocentration ratio of sodium acetate to acetic acid would the indicator show a colour half-way between those of its acid and conjugate base forms? [pKa of acetic acid = 4.75, log 2 = 0.3]

• A. 4 : 1
• B. 6 : 1
• C. 5 : 1
• D. 3 : 1

Answer 1

Answer: (C)

5 : 1

Answer 2

pKa = 5.45

pH = pKHIn + log $\frac{\lbrack Baseform\rbrack}{\lbrack Acideform\rbrack}$ $\Rightarrow$ pH = pKHIn = 5.45 For a Buffer solution

pH = pKa + log $\frac{\lbrack CH_{3}COONa\rbrack}{\lbrack CH_{3}COOH\rbrack} \Rightarrow$ 5.45 = 4.75 + log $\frac{\lbrack CH_{3}COONa\rbrack}{\lbrack CH_{3}COOH\rbrack}$0.7 = log $\frac{\lbrack CH_{3}COONa\rbrack}{\lbrack CH_{3}COOH\rbrack}$

$\Rightarrow$ $\frac{5}{1}$=$\frac{\lbrack CH_{3}COONa\rbrack}{\lbrack CH_{3}COOH\rbrack}$