$A$ buffer solution is formed by a weak acid and its salt with a strong base,or a weak base and its salt with a strong acid. In which of the following pairs is this characteristic found?

The $pH$ of blood is maintained by a proper balance of $H_2CO_3$ and $NaHCO_3$ concentrations. What volume of $5 \, M \, NaHCO_3$ solution should be mixed with $10 \, mL$ of $2 \, M \, H_2CO_3$ to maintain a $pH$ of $7.4$ (in $, mL$)? (Given: $K_a$ for $H_2CO_3 = 7.8 \times 10^{-7}$)

Find the $pH$ of a solution formed by mixing equal volumes of $0.1 \text{ M}$ sodium propionate and $0.1 \text{ M}$ propionic acid. (Given: The dissociation constant of propionic acid is $K_a = 1.3 \times 10^{-5}$)

The acidic buffer $(HX + NaX)$ can be best used to maintain the $pH$ in the range $(4 - 6)$. Find the $pK_b$ of $X^-$.

The $pH$ of blood is maintained constant by the mechanism of:

$A$ buffer solution can be prepared by mixing which of the following?