Question

What fraction of an indicator HIn is in basic form at a pH of 6 if the pKa of the indicator is 5 ?

• A. $\frac{1}{2}$
• B. $\frac{1}{11}$
• C. $\frac{10}{11}$
• D. $\frac{1}{10}$

Answer 1

Answer: (C)

$\frac{10}{11}$

Answer 2

pH = pKa + log $\frac{\lbrack Ionised\rbrack}{\lbrack unlonised\rbrack}$

$\Rightarrow$6 = 5 + log $\frac{\lbrack Ionised\rbrack}{\lbrack unlonised\rbrack} \Rightarrow$1 = log $\frac{\lbrack Ionised\rbrack}{\lbrack unlonised\rbrack}$

$\frac{\lbrack Ionised\rbrack}{\lbrack unlonised\rbrack}$ = 10 $\Rightarrow$ $\frac{\lbrack Ionised\rbrack}{\lbrack Ionised\rbrack + \lbrack unlonised\rbrack}$

= $\left\lbrack \frac{10}{11} \right\rbrack$