pH of polyprotic acid

Chemistry Level 2

Find the pH of $1.0$ M $\text{NaHCO}_{3},$ using the following acid ionization constants: $\begin{aligned} K_{a1} &= \frac{[\text{HCO}_{3}^{-}][\text{H}_{3}\text{O}^{+}]}{[\text{H}_{2}\text{CO}_{3}]}= 4.3 \times 10^{-7}\\ K_{a2} &= \frac{[\text{CO}_{3}^{2-}][\text{H}_{3}\text{O}^{+}]}{[\text{HCO}_{3}^{-}]}= 4.8 \times 10^{-11}. \end{aligned}$

5.66

7.34

7.66

8.34

2 solutions

Swarali Patil
May 4, 2014

Besides going theoretically, I went logically. NaHCO3 is formed by:
NaOH + carbonic acid.
Now as NaOH is a strong base and carbonic acid is weak, the resultant salt: NaHCO3 is basic.
As the other options were nearer to neutral and acidic salts, I chose the most basic pH ,
that is: 8.34
This way, I solved it. :P

$\sqrt{(4.3 \times 10^{-7})(4.8 \times 10^{-11})} \approx 4.54 \times 10^{-9}$

$-\log_{10} (1.0 \times 4.54 \times 10^{-9}) \approx \log_{10} 10^{8.34} = 8.34$ (Logical as basic.)

Formation of $HCO_3^-$ makes an only sequence to proceed for an available answer perhaps.

Answer: $\boxed{8.34}$

Lu Chee Ket - 5 years, 3 months ago

Prateek Bontha
Jul 3, 2014

For a weak dibasic acid, The ph is given by (pKa1+pKa2)/2

Umm no not at all That formula is only valid for an amphiprotic salt

Suhas Sheikh - 3 years ago

pH of polyprotic acid

2 solutions

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