Which of the following solutions does not act as a buffer $:-$

$A$ buffer solution is prepared in which the concentration of $NH_3$ is $0.30 \ M$ and the concentration of $NH_4^+$ is $0.20 \ M.$ If the equilibrium constant,$K_b$ for $NH_3$ equals $1.8 \times 10^{-5},$ what is the $pH$ of this solution? $(log \ 2.7 = 0.43)$

The approximate $pH$ of a solution formed by mixing equal volumes of $0.1 \ M$ sodium propanoate and $0.1 \ M$ propanoic acid (if the dissociation constant of propanoic acid is $1.3 \times 10^{-5}$) will be:

The acidic and basic strength of buffer solutions remains constant because ...

The $pK_b$ of $CN^{\Theta}$ is $4.7$. The $pH$ of a solution prepared by mixing $2.5 \ mol$ of $KCN$ and $2.5 \ mol$ of $HCN$ in water and making the total volume up to $500 \ mL$ is:

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