Which of the following compounds is a Lewis acid?

• A. NH_3
• B. CH_4
• C. BF_3
• D. H_2O

Answer: (C)

Verified step by step guidance

1. Recall the definition of a Lewis acid: a Lewis acid is a species that can accept a pair of electrons to form a coordinate covalent bond.
2. Analyze each compound to determine if it can accept an electron pair: NH_3 (ammonia) has a lone pair and typically donates electrons, so it acts as a Lewis base, not an acid.
3. CH_4 (methane) has no empty orbitals or electron-deficient centers, so it neither accepts nor donates electron pairs easily, making it neither a Lewis acid nor base in typical reactions.
4. BF_3 (boron trifluoride) has an incomplete octet on boron, making it electron-deficient and able to accept a pair of electrons, which classifies it as a Lewis acid.
5. H_2O (water) has lone pairs and usually donates electrons, acting as a Lewis base rather than an acid.