# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: sn1sn2e1e2 on April 19, 2014, 04:21:25 PM

Title: Tricky Acid base question
Post by: sn1sn2e1e2 on April 19, 2014, 04:21:25 PM
Carbonic acid, H2CO3, is a diprotic acid for which K1= 4.2e-7 and K2= 4.7e-11. Which solution will produce a pH closest to 9?

(A) 0.1 M H2CO3
(B) 0.1 M Na2CO3
(C) 0.1 M NaHCO3
(D) 0.1 M NaHCO3 and 0.1 Na2CO3

I know that HCO3- involves amphoteric properties. However, I'm not sure how to approach these kinds. Do you do acid OR base ICE charts? I chose B thinking that the CO32- is the only one with basic properties, hence a pH of 9. However, the correct one is C. Why?? ???
Title: Re: Tricky Acid base question
Post by: Borek on April 19, 2014, 04:42:48 PM
Calculate pH for each case separately.

For C see http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt
Title: Re: Tricky Acid base question
Post by: sn1sn2e1e2 on April 19, 2014, 05:29:01 PM
For A, the pH is 3.69 because you just solve it as a regular acid problem
For B, you just solve it as a regular base problem, ph= 11.66
For D, you solve it has a buffer solution of the second Ka2 acid rendering a ph of 10.33 (Am I right that this is a buffer solution AND you use the pH=pka+log([A-]/[HA])?)

For C, ph is 8.35 because of the plugging in formula.