$0.0100 mol$ of $Na X_{2} CO X_{3}$ is dissolved in water and $0.0120 mol$ of $HCl$ is added; the final volume is $100 mL$ . Given $p K_{a 1} = 6.35$ and $p K_{a 2} = 10.33$ for $H X_{2} CO X_{3}$ and $lo g 2 = 0.301$ , find the pH (two decimals).

Numerical answer type, tolerance 0.01.

Answer: 6.95206

The first

0.0100 mol

H X^{+}

converts

CO X_{3} X^{2 -}

HCO X_{3} X^{-}

; the remaining

0.0020 mol

converts that much

HCO X_{3} X^{-}

H X_{2} CO X_{3}

. Left:

HCO X_{3} X^{-} = 0.0080

H X_{2} CO X_{3} = 0.0020 mol

pH = p K_{a 1} + lo g (0.0080/0.0020) = 6.35 + 0.602 = 6.95