Question

Which of the following set of solutions could be used to prepare a buffer solution?

0.1 M HCl and 0.1 M NaCl
1 M KHCO3 and 1 M KOH
1 M HNO2 and 1 M NaNO3
1 M NaHC2O4 and 1 M H2C2O4

The answer is 1 M NaHC2O4 and 1 M H2C2O4 but why can we use 1 M HNO2 and 1 M NaNO3?

Answer 1

A buffer must be a combination of a weak acid (or base) and a salt that contains the conjugate of that weak acid or base.

Explanation:

While `HNO_2` is a weak acid, the problem with `NaNO_3` is that the `NO_3^-` ion is not the conjugate of a weak acid.

The conjugate of a weak acid will be a base of some appreciable strength which enables it to acquire `H^+` ions to some degree, helping to adjust or control pH, which is the purpose of a buffer.

`NO_3^-` has zero ability to gain `H^+` ions, (since its conjugate acid `HNO_3` is very strong) and will not serve this purpose.

Had the salt been `NaNO_2`, we would have a buffer.

Answer 2

`HNO_2 and NaNO_3` do not make up a required "weak acid and salt" pair, as `NaNO_3` is not a resulting salt or conjugate base of `HNO_2` neutralization.

Explanation:

A buffer could be made with `HNO_2 and NaNO_2` in solution.
bilbo.chm.uri.edu/CHM112/lectures/buffer.htm

A buffer is a solution that can maintain a nearly constant pH if it is diluted, or if relatively small amounts of strong acids or bases are added.

A buffer solution can be made by mixing a weak acid with one of its salts OR mixing a weak base with one of its salts.

A more technical way of saying this is that a buffer solution consists of a mixture of a weak acid and its conjugate base OR a weak base and its conjugate acid.
https://www.chemicool.com/definition/buffers_acid_base.html

And:
https://www.thoughtco.com/definition-of-buffer-604393