Name: pH of a Solution
Uploaded: 2019-07-19
Duration: 4 min 31 s
Description: This video discusses the pH scale and how it is useful to determine the behaviour of an acidic or basic solution. Does it also tell what the pH of a neutral...

Question

Write a note on ionisation of polybasic acid with example.

Accepted Answer

Acids capable of yielding more than one hydronium (or hydrogen ion) ions per molecule are called polybasic acids. The terms dibasic, tribasic, etc. are indicating the number of replaceable hydrogen atoms.

Example: Sulphuric acid, phosphoric acid, carbonic acid etc.

Ionisation of carbonic acid:

Carbonic acid has the following equilibria in aqueous solutions:

$H_{2} {CO}_{3} (aq) + H_{2} O (l) \overset{}{⇌} H_{3} O^{+} (aq) + {HCO}_{3}^{-} (aq) : K_{a_{1}}$

${HCO}_{3}^{-} (aq) + H_{2} O (l) \overset{}{⇌} H_{3} O^{+} (aq) + {CO}_{3}^{2 -} (aq) : K_{a_{2}}$

The overall reaction is, $H_{2} {CO}_{3} (aq) + 2 H_{2} O (l) \overset{}{⇌} 2 H_{3} O^{+} (aq) + {CO}_{3}^{2 -} (aq) : K_{a}$

$K_{a} = \frac{{[H_{3} O^{+}]}^{2} [{CO}_{3}^{2 -}]}{[H_{2} {CO}_{3}]} = \frac{{[H_{3} O^{+}]}^{2} [{CO}_{3}^{2 -}] [{HCO}_{3}^{-}]}{[H_{2} {CO}_{3}] [{HCO}_{3}^{-}]} = K_{a_{1}} \times K_{a_{2}}$

Therefore, $K_{a} = K_{a_{1}} \times K_{a_{2}}$ Where $K_{a}$ is the dissociation or ionisation constant of the acid.

Write a note on ionisation of polybasic acid with example.

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