Which of the following is not a lewis acid?

Solution:

Electrons can be taken up by Lewis Acid. Thus, an acidic substance is often one that has a positive charge. Furthermore, a compound having vacant orbitals.

An electron donor is a Lewis base. In general, a combination having a negative charge would be referred to as a base. Furthermore, a molecule containing lone pairs.

Lewis acids include ${\mathrm{BF}}_3$, ${\mathrm{AlCl}}_3$, and ${\mathrm{FeCl}}_3$ because they all possess empty orbitals that may take electron pairs.

Lewis acids do not include ${\mathrm{PH}}_3$. It can utilise a pair of electrons from the pi bond to donate to a Lewis acid, making it a Lewis base.