Question

The pH water at $25{}^{0}C$ and $35{}^{0}C$ is 7 and 6 respectively. The heat of formation of water is:

• A. -84.5 k.cal
• B. 84.5 kcal
• C. -54.7 k.cal
• D. 54.7 k.cal

Answer 1

The correct option is A -84.5 k.cal

At ${25}^{o}C,\left[H^{+}\right]={10}^{-7}{K}_w={10}^{-14}$

At ${35}^{o}C,\left[H^{+}\right]={10}^{-6}{K}_w={10}^{-12}$

$2.303\log \frac{K_{w_2}}{K_{w_1}}=\frac{\Delta H}{R}\left[\frac{T_2-T_1}{T_1{T}_2}\right]$

$2.303\log \frac{{10}^{-12}}{{10}^{-14}}=\frac{\Delta H}{2}\left[\frac{10}{298\times 308}\right]$

$\Delta H=84551.4calmo{l}^{-1}$

$=84.55k.calm{ol}^{-1}$

i. e., $H_{2}O\rightleftharpoons H^{+}+O{H}^{-};\Delta H=84.55k.cal$

For $H^{+}+O{H}^{-}\rightleftharpoons H_{2}O;\Delta H=-84.55k.cal$

Hence, the correct option is $\text{A}$