One litre of a buffer solution containing 0.02 mol of propanoic acid and some sodium propanate has pH = 4.75. What will be the pH if 0.01 mol of hydrogen chloride is dissolved in the above buffer solution ? [Dissociation constant of propanoic acid at 25^(@)C is 1.34 xx 10^(-5)].

One litre of a buffer solution containing 0.02 mol of propanoic acid a

1.	One litre of a buffer solution containing 0.02 mol of propanoic acid and some sodium propanate has pH = 4.75. What will be the pH if 0.01 mol of hydrogen chloride is dissolved in the above buffer solution ? [Dissociation constant of propanoic acid at 25^(@)C is 1.34 xx 10^(-5)].
Answer» `4.11` `2.11` `0.11` `3.11` Solution :Using the expression `PH = pKa + "log" (["salt"])/(["acid"])` We get `4.75 = - log(1.34 xx 10^(-5)) + "log" (["salt"])/(["acid"])` which gives `4.75 = 4.87 + "log" (["salt"])/(0.02M)` or `(["Salt"])/(0.02M) = 0.76` or `["salt"] = 1.52 xx 10^(-2)M` Hence, AMOUNT of sodium propanoate to be added `= 1.52 xx 10^(-2) "mol"` The addition of `0.01` hydrogen chloride convert the equivalent amount of sodium propanoate into PROPANOIC acid.