- #1

Titan97

Gold Member

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My teacher was teaching me salt hydrolysis today and I understood everything up to hydrolysis of salt of weak acid and weak base. Then he introduced amphiprotic anion in salt.

He took NaHCO

This is what he wrote:

HCO

H

HCO

$$k_{a1}=\frac{[H^+][HCO_3^{-}]}{[H_2CO_3]}$$

$$k_{a2}=\frac{[H^+][CO_3^{2-}]}{[HCO_3^{-}]}$$

He then assumed that concentration of carbonate ion and concentration of

H

$$pH=\frac{1}{2}(pk_{a1}+pk_{a2})$$

I don't understand that assumption. Also, why did he assume that concentration of hydronium ions in both first and second step to be equal? Is the final result correct? Where can I read more of this? He also discussed about salts where both ions are amphiprotic. I did not understand anything.

He took NaHCO

_{3}This is what he wrote:

HCO

_{3}^{-}+H_{2}O ##\leftrightarrows## H_{2}CO_{3}+OH^{-}H

_{2}CO_{3}##\leftrightarrows##H^{+}+HCO_{3}^{-}HCO

_{3}^{-}##\leftrightarrows##CO_{3}^{2-}+H^{+}$$k_{a1}=\frac{[H^+][HCO_3^{-}]}{[H_2CO_3]}$$

$$k_{a2}=\frac{[H^+][CO_3^{2-}]}{[HCO_3^{-}]}$$

He then assumed that concentration of carbonate ion and concentration of

H

_{2}CO_{3}to be same and after multiplying the k_{a}values he got$$pH=\frac{1}{2}(pk_{a1}+pk_{a2})$$

I don't understand that assumption. Also, why did he assume that concentration of hydronium ions in both first and second step to be equal? Is the final result correct? Where can I read more of this? He also discussed about salts where both ions are amphiprotic. I did not understand anything.

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