Which of the following compounds accepts a proton from a water molecule according to the Bronsted-Lowry theory?

Which one of the following substances has the highest proton affinity?

The $pK_a$ of acetylsalicylic acid (aspirin) is $3.5$. If the $pH$ of human stomach fluid is $2-3$ and the $pH$ of the small intestine fluid is $8$,then aspirin will:

Which of the following is not a conjugate acid-base pair?

The number of species among the following that can act both as Bronsted acids and bases is $HCl, ClO_4^{-}, OH^{-}, H^{+}, H_2O, HSO_4^{-}, SO_4^{2-}, H_2SO_4, Cl^{-}$.

What is a Lewis acid?