The compound shown is treated with aqueous sodium hydrogen carbonate (the conjugate acid, carbonic acid, has pKa1 about 6.4); only an acidic group with pKa clearly below 6.4 is deprotonated. Take the pKa of a sulfonic acid as below 0, a benzoic acid as about 3 to 4 and a phenol as about 8 to 13. How many acidic hydrogen atoms of the compound are removed? Give the answer as an integer.

Numerical answer type, tolerance 0.5.

Answer: 2

The compound is 5-sulfosalicylic acid. The SO3H group (pKa < 0) and the COOH group (pKa about 3) are stronger than carbonic acid and react with NaHCO3. The phenolic OH (pKa about 10 or higher, all phenols being above 6.4) is weaker than carbonic acid and is not deprotonated. Number = 2.

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Answer: 2

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