Which of the following cannot act as a buffer solution?

The $pH$ of an aqueous solution containing $1 \ M$ benzoic acid $(pK_{a}=4.20)$ and $1 \ M$ sodium benzoate is $4.5$. The volume of benzoic acid solution in $300 \ mL$ of this buffer solution is . . . . . . $mL$.

$A$ buffer solution with $pH = 9$ is to be prepared by mixing $NH_4Cl$ and $NH_4OH$. Calculate the number of moles of $NH_4Cl$ that should be added to one litre of $1.0 \ M \ NH_4OH$. $[K_b = 1.8 \times 10^{-5}]$

$50 \, mL$ of $2 \, N$ acetic acid mixed with $10 \, mL$ of $1 \, N$ sodium acetate solution will have an approximate $pH$ of $(K_a = 10^{-5})$

Which of the following mixtures in water acts as a buffer?

$A$ weak acid with a dissociation constant of $10^{-5}$ is being titrated with an aqueous $NaOH$ solution. The $pH$ at the point of one-third neutralization of the acid will be: