Which of the following will not function as a buffer solution?

Which of the following mixtures will have the lowest $pH$ at $298 \ K$?

In order to prepare a buffer solution of $pH = 5.74$,sodium acetate is added to acetic acid. If the concentration of acetic acid in the buffer is $1.0 \ M$,the concentration of sodium acetate in the buffer is $......... \ M$. (Round off to the nearest integer).
[Given: $pKa$ (acetic acid) $= 4.74$]

The Henderson-Hasselbalch equation for an acidic buffer is given by $pH - pK_a = 5$. This condition holds true when:

Which of the following is not a buffer solution?

The $pH$ of a buffer solution obtained by mixing $50 \ mL$ of $0.1 \ M$ $NH_4OH$ and $25 \ mL$ of $2.0 \ M$ $NH_4Cl$ is ($pK_b$ of $NH_4OH$ is $4.8$).