Ncert Exercises & Solutions

On the basis of the equation $pH = - \log [H^{+}]$ , the pH of $10^{- 8} mol {dm}^{- 3}$ solution of HCl should be 8. However, it is observed to be less than 7.0. Explain the reason.

Concentration

10^{- 8} mol {dm}^{- 3}

indicates that the solution is very dilute. So, we cannot neglect the contribution of

H_{3} O^{+}

ions produced from

H_{2} O

in the solution. Total

[H_{3} O^{+}] = 10^{- 8} + 10^{- 7} M

. From this we get the value of pH close to 7 but less than 7 because the solution is acidic.

From calculation, it is found that pH of

10^{- 8} mol {dm}^{- 3}

solution of HCl is equal to 6.96.

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