Ncert Exercises & Solutions

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

(1) Equal to 7.0

(2) Greater than 7.0

(3) Less than 7.0

(4) Equal to zero

(3) HINT: With increase in temperature K_w increases.

The pH of neutral water at

25 ° C

is 7.0.

25 ° C, [H^{+}] = [{OH}^{-}] = 10^{- 7}

and

K_{w} = [H^{+}] [{OH}^{-}] = 10^{- 14}

On heating, K_w increases, i.e.,

[H^{+}] [{OH}^{-}] > 10^{- 14}

[H^{+}] = {[OH]}^{-} or, {[H^{+}]}^{2} > = 10^{- 14}

or,

[H^{+}] > 10^{- 7} M

∴ pH < 7

With rise in temperature, pH of pure water decreases and it become less than 7 at

60 ° C

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