Which of the following is not a Lewis acid?

Arrange $NH_4^+$,$H_2O$,$H_3O^+$,$HF$,and $OH^-$ in increasing order of their acidic strength.

Derive $K_w = K_a \times K_b$ and $pK_w = pK_a + pK_b$ for a weak base $B$ and its conjugate acid $BH^{+}$.

$CH_3COOH_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + CH_3COO^{-}_{(aq)}$. Between $H_2O$ and $CH_3COO^{-}$,which one is the weaker base?

$pK_a$ values of two acids $A$ and $B$ are $4$ and $5$. The strengths of these two acids are related as:

Which of the following cannot act both as Bronsted acid and as Bronsted base?