Which of the following solutions does not act as a buffer?

Aqueous solutions of $HNO_{3}$,$KOH$,$CH_{3}COOH$,and $CH_{3}COONa$ of identical concentration are provided. The pair$(s)$ of solutions which form a buffer upon mixing is (are):

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

Which of the following acts as a buffer solution?

At $298 \text{ K}$,a certain buffer solution contains equal concentrations of $X^-$ and $HX$. The $K_b$ for $X^-$ is $10^{-10}$. What is the $pH$ of this buffer solution?

$20 \ mL$ of $0.1 \ M$ acetic acid is mixed with $50 \ mL$ of potassium acetate. $K_a$ of acetic acid $= 1.8 \times 10^{-5}$ at $27^{\circ} C$. Calculate the concentration of potassium acetate if the $pH$ of the mixture is $4.8$. (in $M$)