The $pH$ of blood is

Which of the following buffers is used to maintain the $pH$ of human blood naturally?

What is the ratio of concentration of salt to concentration of weak acid in a buffer solution to maintain its $pH$ value at $7.2$ $(pK_{a} = 6.2)$?

Calculate the $pH$ of the solution in which $0.2 \, M \, NH_{4}Cl$ and $0.1 \, M \, NH_{3}$ are present. The $pK_{b}$ of ammonia solution is $4.75$.

Which of the following is a buffer solution?

The $pK_a$ of $HCN$ is $9.30$. What is the $pH$ of a solution prepared by mixing $2.5 \ mol$ of $KCN$ and $2.5 \ mol$ of $HCN$ in $500 \ mL$ of water (in $.30$)?