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Question
The dissociation constant of a weak acid is 1 × 10−3. In order to prepare a buffer solution with a pH = 4, the `(["Acid"])/(["Salt"])` ratio should be ____________.
Options
4 : 3
3 : 4
10 : 1
1 : 10
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Solution
The dissociation constant of a weak acid is 1 × 10−3. In order to prepare a buffer solution with a pH = 4, the `(["Acid"])/(["Salt"])` ratio should be 1 : 10.
Explanation:
Ka = 1 × 10−3
pH = 4
`(["Salt"])/(["Acid"])` = ?
pH = `"pK"_"a" + log (["Salt"])/(["Acid"])`
4 = `-log_10 (1 xx 10^-3) + log (["Salt"])/(["Acid"])`
4 = `3 + log_10 (["Salt"])/(["Acid"])`
1 = `log_10 (["Salt"])/(["Acid"])`
`(["Salt"])/(["Acid"])` = 101
i.e., `(["Acid"])/(["Salt"]) = 1/10` = 1 : 10
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