Which of the following compounds has the lowest $pKa$ value?

• A. ${\mathrm{CH}}_4$ (methane)
• B. ${\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid)
• C. ${\mathrm{NH}}_3$ (ammonia)
• D. ${\mathrm{CH}}_3\mathrm{OH}$ (methanol)

Answer: (B)

Verified step by step guidance

1. Understand that the pKa value is a measure of the acidity of a compound; the lower the pKa, the stronger the acid.
2. Recall the typical pKa ranges for the given compounds: methane (around 50), ammonia (around 38), methanol (around 16), and acetic acid (around 4.7).
3. Recognize that methane is very weakly acidic due to the strong C-H bond and lack of resonance stabilization of its conjugate base.
4. Note that ammonia is more acidic than methane but still much less acidic than alcohols and carboxylic acids because the conjugate base (amide ion) is not well stabilized.
5. Identify that acetic acid has the lowest pKa because its conjugate base (acetate ion) is resonance stabilized, making it the strongest acid among the options.