Question

The pH of neutral water at $25^{o}C$ is $7.0$As the temperature increases, ionisation of water increases, however, the concentration of $H^{+}$ ions and $OH^{-}$ ins are equal. What will be the pH of pure water at $60^{o}C?$

• A. Equal to $7.0$
• B. Greater than $7.0$
• C. Less than $7.0$
• D. Equal to zero

Answer 1

Answer: (C)

Less than $7.0$

Answer 2

: The pH of neutral water at $25^{o}C$ is $7.0.$

$\therefore\lbrack H^{+}\rbrack = \lbrack OH^{-}\rbrack = 10^{- 7}$ $(\because pH = - \log\lbrack H^{+}\rbrack)$

Now, $K_{w} = \lbrack H^{+}\rbrack\lbrack OH^{-}\rbrack = (1 \times 10^{- 7})^{2} = 1 \times 10^{- 14}$

As the temperature increases, ionisation of water increases, thus $\lbrack H^{+}\rbrack$ and $\lbrack OH^{-}\rbrack$ increases equally. Now

$K_{W} = \lbrack H^{+}\rbrack\lbrack OH^{-}\rbrack > 1 \times 10^{- 14}$ $(\because\lbrack H^{+}\rbrack = \lbrack OH^{-}\rbrack)$

or $\lbrack H^{+}\rbrack^{2} > 1 \times 10^{- 14}$

$\therefore\lbrack H^{+}\rbrack > 1 \times 10^{- 7}$ and pH < 7