Which of the followings are Lewis acids?
Question:

Which of the followings are Lewis acids? $\mathrm{H}_{2} \mathrm{O}, \mathrm{BF}_{3}, \mathrm{H}^{+}$, and $\mathrm{NH}_{4}^{+}$

Solution:

Lewis acids are those acids which can accept a pair of electrons. For example, $\mathrm{BF}_{3}, \mathrm{H}^{+}$, and $\mathrm{NH}_{4}^{+}$are Lewis acids.