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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3sb218

Use pKa1 = 6.35 and pKa2 = 10.33 for $H X_{2} CO X_{3}$ at 298 K and pKw = 14. Which of the following statements about 0.10 M aqueous solutions are correct?

AThe pH of $NaHCO X_{3}$ solution is about 8.3.
BThe pH of $Na X_{2} CO X_{3}$ solution is close to 11.65, if hydrolysis to $H X_{2} CO X_{3}$ is ignored.
CThe order of pH is $NaOH > Na X_{2} CO X_{3} > NaHCO X_{3}$ .
D $NaHCO X_{3}$ solution is acidic because $HCO X_{3} X^{-}$ can donate a proton.

Answer: A, B, C

For the amphiprotic ion,

pH \approx (p K_{a 1} + p K_{a 2}) /2 = 8.34

. For carbonate,

K_{b} = 1 0^{- 14} /1 0^{- 10.33} = 2.14 e - 04

, and

x^{2} / (0.10 - x) = K_{b}

gives

[OH X^{-}] = 4.52 e - 03

M, pH

= 11.65

. 0.10 M NaOH has pH 13, so the order holds. A pH of 8.3 is alkaline, so the last statement is false.

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