Question

The indicator constant for an acidic indicator , HIn is $5\times {10}^{-6}M$. this indicator appears only in the colour of acidic from when $\frac{\left[I{n}^{-}\right]}{\left[HIn\right]}\le \frac{1}{20}$ and it appears only in the colour of basic form when $\frac{\left[HIn\right]}{\left[I{n}^{-}\right]}\le \frac{1}{40}.$ the pH range of indicator is:

• A. 3.7 - 6.9
• B. 4.0 - 6.6
• C. 4.0 -6.9
• D. 3.7 - 6.6

Answer 1

The correct option is C 4.0 -6.9

$pH=pKIn+lo{g}_{10}\left[\frac{I{n}^{-}}{HIn}\right]$

$pH=[6-lo{g}_{10}5]+lo{g}_{10}\left[\frac{1}{20}\right]=4$

$pH=pKIn+lo{g}_{10}\left[\frac{I{n}^{-}}{HIn}\right]$

$=pKIn+lo{g}_{10}\left[\frac{40}{1}\right]=6-lo{g}_{10}5+lo{g}_{10}40=6-0.7+1.6=6.9$