10th question

10.  At what temperature will water act as acid if K_W value of water at different temperatures is as follows:

$$\begin{gathered} \left(\mathrm{A}\right) {\mathrm{K}}_{\mathrm{W}} = {10}^{-14} (\mathrm{at} 25°\mathrm{C}) \\ \left(\mathrm{B}\right) {\mathrm{K}}_{\mathrm{W}} = {10}^{-10} (\mathrm{at} 80°\mathrm{C}) \\ \left(\mathrm{C}\right) {\mathrm{K}}_{\mathrm{W}} = {10}^{-16} (\mathrm{at} 4°\mathrm{C}) \\ \left(\mathrm{D}\right) \mathrm{None} \mathrm{of} \mathrm{these}. \end{gathered}$$
 

 

Dear Student

The dissociation of water, H₂O is represented as -
      H₂O  $\leftrightarrow$    H⁺   +   OH^-

According to the Law of Mass Action -
k = $\frac{\left[H^{+}\right] \left[O{H}^{-}\right]}{\left[H_{2}O\right]}$
where, k is the constant and the concentrations are represented in molarity.

Since, the degree of dissociation is very small, therefore the concentration of undissociated molecules of water, H₂O can also be taken as a constant, k_w -
k_w = [H⁺] [OH^-]

This value of dissociation constant of water, k_w is temperature dependent, and at 25^oC, its value is equal to 10^-14 .

So, the correct option is (a).

Regards