Which one of the following is not a buffer solution?

Which of the following mixtures forms a buffer solution?

What is the $pH$ of the solution obtained by mixing $5.076 \ g$ of methyl ammonium nitrate $(CH_3NH_3NO_3)$ into $120 \ mL$ of $0.225 \ M$ methylamine $(CH_3NH_2$; $K_b = 4 \times 10^{-4})$?

Buffer Solution	Volume of $0.1 \ M$ Weak Acid (mL)	Volume of $0.1 \ M$ Sodium Salt (mL)
$I$	$4.0$	$4.0$
$II$	$4.0$	$40.0$
$III$	$40.0$	$4.0$
$IV$	$0.1$	$10.0$

Which of the two sets of buffer solutions have the least $pH$?

$A$ buffer solution is prepared by mixing $0.2 \ M$ sodium acetate and $0.1 \ M$ acetic acid. If $pK_{a}$ for acetic acid is $4.7$,find the $pH$.

$A$ buffer solution is prepared by mixing $10 \ mL$ of $1.0 \ M$ acetic acid and $20 \ mL$ of $0.5 \ M$ sodium acetate and then diluted to $100 \ mL$ with distilled water. The $pH$ of the buffer solution is ($pK_a$ of acetic acid is $4.76$).