Which salt can be classified as an acid salt?

$K_{a}$ values for acids $H_{2}SO_{3}$,$HNO_{2}$,$CH_{3}COOH$ and $HCN$ are respectively $1.3 \times 10^{-2}$,$4 \times 10^{-4}$,$1.8 \times 10^{-5}$ and $4 \times 10^{-10}$. Which of the above acids produces a stronger conjugate base in aqueous solution?

$NaOH$ is a strong base because......

Which of the following cannot act as a Lewis or Bronsted acid?

The conjugate base of $NH_2^-$ is

Which of the following is not a Lewis acid?