What is a Lewis acid?

Which of the following compounds is not basic?

The position of equilibrium lies to the right in each of these reactions:
$N_2H_5^+ + NH_3 \rightleftharpoons NH_4^+ + N_2H_4$
$NH_3 + HBr \rightleftharpoons NH_4^+ + Br^{-}$
$N_2H_4 + HBr \rightleftharpoons N_2H_5^+ + Br^{-}$
Based on this information,what is the order of acidic strength?

Which of the following equilibria represents a $Lewis$ acid-base reaction but not a $Br\o nsted$ acid-base reaction?

Which of the following can give $OH^{-}$ ions?

When ${K_2}O$ is added to water,the solution becomes basic because it contains which of the following?