ii. Define pH and pOH. Derive relationship between pH and pOH.

4. Answer the following :

Answer:-

Instead of writing concentration of H3O⊕ ions in mol dm-3, sometimes it is convenient to express it on the logarithmic scale. This is known as pH scale.

pOH of a solution can be defined as the negative logarithm to the base 10, of the molar concentration of OH ions in solution.

Relationship between pH and pOH:

The ionic product of water is given as:

\(K_w = [H_3O^+][OH^-]\)

Now, \(K_w = 1 \times 10^{-14}\) at 298 K

Thus, \([H_3O^+][OH^-] = 1.0 \times 10^{-14}\)

Taking the logarithm of both sides, we write

\(\log_{10}[H_3O^+] + \log_{10}[OH^-] = -14\)

\(-\log_{10}[H_3O^+] + (-\log_{10}[OH^-]) = 14\)

Now, \(pH = -\log_{10}[H_3O^+]\) and \(pOH = -\log_{10}[OH^-]\)

\(\therefore pH + pOH = 14\)