Regular use of which of the following fertilizers increases the acidity of the soil?

The aqueous solution of $FeCl_3$ is acidic in nature. This is due to:

Which of the following salts undergoes hydrolysis in water?

The degree of hydrolysis of a $N/100$ $KCN$ solution is = ....... (Given $K_a = 1.4 \times 10^{-9}$)

For a weak acid $HA$,$pK_a = 4.80$. For a weak base $BOH$,$pK_b = 4.78$. What is the $pH$ of an aqueous solution of the salt $BA$?

$20\,mL$ of $0.1\,M\,NH_{4}OH$ is mixed with $40\,mL$ of $0.05\,M\,HCl$. The $pH$ of the mixture is nearest to. (Given: $K_{b}(NH_{4}OH) = 1 \times 10^{-5}, \log 2 = 0.30, \log 3 = 0.48, \log 5 = 0.69, \log 7 = 0.84, \log 11 = 1.04$) (in $.2$)