Question

Calculate the pH range in which an acid indicator with $K_{acid}$(indicator) $=1.0\times {10}^{-5}$ changes colour when the concentration of the indicator is $1\times {10}^{-3}$ M.

• A. $5\pm 1$
• B. $11\pm 1$
• C. $3\pm 1$
• D. $8\pm 1$

Answer 1

The correct option is A $5\pm 1$

$P^{H}range=(P^{K_{In}}-1)to(P^{K_{In}}+1)$

$HIn\rightleftharpoons H^{+}(aq.)+I{n}^{+}(aq.);Ka=KIn$

$KIn=1\times {10}^{-5}\Rightarrow P^{K_{In}}=5$

$P^{H}range=5\pm 1$