Which of the following is the strongest acid in water?

A $HC \equiv CH$
B $C X_{2} H X_{5} OH$
C $C X_{6} H X_{5} OH$
D $CH X_{3} COOH$

Answer: D

Approximate pKa values: acetic acid 4.76, phenol 10.0, ethanol 15.9, ethyne 25. Acetic acid has the lowest pKa because the acetate ion is resonance stabilised with the charge shared equally by two electronegative oxygen atoms. Hence acetic acid is the strongest acid.

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Which of the following is the strongest acid in water?

Answer: D

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