- #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 NaHCO3
This is what he wrote:
HCO3-+H2O ##\leftrightarrows## H2CO3+OH-
H2CO3##\leftrightarrows##H++HCO3-
HCO3-##\leftrightarrows##CO32-+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
H2CO3 to be same and after multiplying the ka 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.
He took NaHCO3
This is what he wrote:
HCO3-+H2O ##\leftrightarrows## H2CO3+OH-
H2CO3##\leftrightarrows##H++HCO3-
HCO3-##\leftrightarrows##CO32-+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
H2CO3 to be same and after multiplying the ka 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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