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Topic: Tricky Acid base question  (Read 4292 times)

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Offline sn1sn2e1e2

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Tricky Acid base question
« 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?? ???

Offline Borek

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Re: Tricky Acid base question
« Reply #1 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
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline sn1sn2e1e2

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Re: Tricky Acid base question
« Reply #2 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.

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