Step 1: Understanding the Concept:
This question tests the understanding of the Lewis acid-base theory.
A Lewis acid is a chemical species that can accept a pair of electrons. Common examples include species with an incomplete octet of electrons or cations with vacant orbitals.
A Lewis base is a chemical species that can donate a pair of electrons. Common examples include species with lone pairs of electrons or anions.
We need to analyze each species in the given pairs to determine if they are Lewis acids or bases.
Step 2: Key Formula or Approach:
Check each species for the following characteristics:
- For Lewis acids: Incomplete octet, vacant orbitals, or positive charge (cation).
- For Lewis bases: Lone pair of electrons or negative charge (anion).
Step 3: Detailed Explanation:
Let's analyze the pairs:
(A) BF$_3$, NH$_3$:
BF$_3$: Boron in BF$_3$ has only 6 valence electrons, an incomplete octet. It can accept an electron pair to complete its octet, making it a Lewis acid.
NH$_3$: Nitrogen in ammonia has a lone pair of electrons which it can donate, making it a Lewis base.
This pair contains a Lewis acid and a Lewis base.
(B) H$^+$, BF$_3$:
H$^+$: The proton has an empty 1s orbital and readily accepts an electron pair, making it a Lewis acid.
BF$_3$: As established above, it is a Lewis acid.
This pair contains only Lewis acids.
(C) F$^-$, H$_2$O:
F$^-$: The fluoride ion has four lone pairs of electrons and can donate an electron pair, making it a Lewis base.
H$_2$O: The oxygen atom in water has two lone pairs of electrons, making it a Lewis base.
This pair contains only Lewis bases.
(D) NH$_4^+$, NH$_3$:
NH$_4^+$: The ammonium ion has no lone pairs and a full octet on nitrogen. It acts as a Brønsted-Lowry acid (proton donor) but not typically as a Lewis acid (electron pair acceptor).
NH$_3$: As established above, it is a Lewis base.
This pair does not contain only Lewis acids.
Step 4: Final Answer:
The pair containing only Lewis acids is H$^+$ and BF$_3$. Therefore, option (B) is correct.