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

Answer 1

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:

K_a = 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