The $pH$ of blood does not appreciably change by a small addition of acid or a base because blood

$A$ buffer solution of $100 \ mL$ has a $pH$ value of $4$. When $1 \ mL$ of dilute $HCl$ is added to it,what happens to the $pH$ of the buffer solution?

$3 \; g$ of acetic acid is added to $250 \; mL$ of $0.1 \; M \; HCl$ and the solution is made up to $500 \; mL$. To $20 \; mL$ of this solution,$\frac{1}{2} \; mL$ of $5 \; M \; NaOH$ is added. The $pH$ of the solution is: [Given: $pK_{a}$ of acetic acid $= 4.75$,molar mass of acetic acid $= 60 \; g/mol$,$\log 3 = 0.4771$]. Neglect any changes in volume.

Derive the Henderson-Hasselbalch equation.

The $pH$ of a buffer solution obtained by mixing $50 \ mL$ of $0.1 \ M$ $NH_4OH$ and $25 \ mL$ of $2.0 \ M$ $NH_4Cl$ is ($pK_b$ of $NH_4OH$ is $4.8$).

For preparing a buffer solution of $pH = 6$ by mixing sodium acetate and acetic acid,the ratio of the concentration of salt and acid should be $(K_a = 10^{-5})$