Q. By the addition of 1.2 mole of $H_{2}CO_{3}$ and 0.4 mole of $Na_{2}CO_{3}$ solution is formed. $\left(\right.K_{a_{1}}=4\times \left(10\right)^{- 6}, \, K_{a_{2}}=4\times \left(10\right)^{- 11}\left.\right)$ . Identify the pH of 1 L solution.

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A

5.4

B

7.4

C

9

D

5.8

Solution:

$H_{2}CO_{3} & + & Na_{2}CO_{3} & \rightarrow & 2NaHCO_{3} \\ 1.2 & & 0.4 & & 0 \\ 0.8 & & 0 & & 0.8$
$pH=-log\left(\right.4\times \left(10\right)^{- 6}\left.\right)+log\frac{0 .8 / 1}{0 .8 / 1}$
pH = 6 - log4 = 5.4

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