Consider the following: I. AlCl_3 II. NH_3 III. BF_3 IV. FeCl_3 How...

Lewis acids are chemical species that can accept a pair of electrons. They are typically electron-deficient molecules or cations with empty orbitals.

$\text{AlCl}_3$ has an incomplete octet (6 electrons around Al) and acts as a Lewis acid.

$\text{NH}_3$ has a lone pair of electrons on the nitrogen atom and acts as a Lewis base (electron pair donor).

$\text{BF}_3$ has an incomplete octet (6 electrons around B) and acts as a Lewis acid.

$\text{FeCl}_3$ has empty d-orbitals on the central iron atom, allowing it to accept electron pairs, thus acting as a Lewis acid.

Therefore, $\text{AlCl}_3$, $\text{BF}_3$, and $\text{FeCl}_3$ are Lewis acids. There are three Lewis acids in the given list.

Answer: Three