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JEE Main levelSingle correct optionDifficulty 2 of 3sb136

For carbonic acid $p K_{a 1} = 6.35$ and $p K_{a 2} = 10.33$ at 298 K. The pH of a moderately concentrated aqueous solution of $NaHCO X_{3}$ is approximately:

A8.34
B6.35
C10.33
D7.00

Answer: A

HCO X_{3} X^{-}

is amphiprotic, so

pH \approx \frac{1}{2} (p K_{a 1} + p K_{a 2})

. This gives

\frac{1}{2} (6.35 + 10.33) = 8.34

.

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