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JEE Advanced levelNumerical answerDifficulty 3 of 3bm083

Alanine has $p K_{a 1} = 2.34$ . Ignoring the anionic form, at what pH is the ratio of zwitterion to cation equal to 20 : 1? Give the answer to two decimal places (use $lo g 2 = 0.3010$ ).

Numerical answer type, tolerance 0.02.

Answer: 3.64

For

H X_{3} N X^{+} - CHR - COOH ⇌ H X_{3} N X^{+} - CHR - COO X^{-}

,

p H = p K_{a 1} + lo g ([zwitterion] / [cation]) = 2.34 + lo g 20

.

lo g 20 = 1 + 0.3010 = 1.3010

, so pH =

3.641 \approx 3.64

.

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