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JEE Main levelSingle correct optionDifficulty 2 of 3bm063

In strongly acidic solution (pH about 1) alanine exists predominantly as

A $H X_{2} N - CH (CH X_{3}) - COO X^{-}$
B $H X_{3} N X^{+} - CH (CH X_{3}) - COOH$
C $H X_{2} N - CH (CH X_{3}) - COOH$
D $H X_{3} N X^{+} - CH (CH X_{3}) - COO X^{-}$

Answer: B

At very low pH both basic and acidic groups are protonated: the amino group is

- NH X_{3} X^{+}

and the carboxyl is

- COOH

, giving a cation of net charge

+ 1

. The zwitterion forms near neutral pH and the anion in alkaline solution.

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