Which of the following solutions cannot act as a buffer?

During the titration of $N/10$ acetic acid with $N/10 \, NaOH$,what is the $pH$ of the solution when $25\%$,$50\%$,and $75\%$ of the titration is complete? $[K_a = 10^{-5}]$

What is the $OH^-$ ion concentration of a solution containing $0.05 \, M$ ammonium hydroxide and $0.001 \, M$ ammonium chloride? Given $K_b(NH_4OH) = 1.8 \times 10^{-5}$.

$pH$ of a solution of $10 \ mL$ $1 \ N$ sodium acetate and $50 \ mL$ $2 \ N$ acetic acid $(K_a = 1.8 \times 10^{-5})$ is approximately

$0.1 \ M, 60 \ mL$ $NH_4Cl$ $(K_h = 10^{-9})$ is titrated against $0.2 \ M$ $NaOH$. What is the $pH$ at the $\frac{1}{3}$ equivalence point?

What would be the $pH$ of a solution obtained by mixing $5 \ g$ of acetic acid and $7.5 \ g$ of sodium acetate and making the volume equal to $500 \ mL$? $(K_a = 1.75 \times 10^{-5}, pK_a = 4.76)$