1. (2) At pH 5.0, what is the concentration of OH - in solution? ___________ 2.
ID: 914375 • Letter: 1
Question
1. (2) At pH 5.0, what is the concentration of OH- in solution? ___________
2. (2) If the monoprotic acid HAb has a pK of 3 and a solution is adjusted to pH 5:
a) what is the concentration of hydrogen ions in that solution? ____________
b) what is the ratio of Ab- to HAb? ________________
3. (2) Oxalic acid, a di-protic, di-carboxylic acid has pKs of 1.3 and 4.3 and thus can exist as H2Ox, HOx- and Ox= (please use the same in your answer) .
a) What would be the dominant form at pH 3.3? ________________
b) What would be the second most dominant from at pH 3.3? ___________
Explanation / Answer
1)At pH = 5, the hydrogen ion concentration is [H+] = 1 x 10-5M thus [OH-] = Kw / [H+]
[OH-] = 1.0x 10-14 / 1.0 x 10-5 = 1.0 x 10 -9 M
2) pKa of the3 acid = 3.0 which shows it is relatively weak acid.
HA <---> H+ +A- and 1.0x 10-3 = Ka = [H+] [A-] / [HA]
a) When the pH = 5.0 , the [H+} = 1.0x 10-5 M as the pH is adjusted to pH= 5.0
b) the ratio [A-] / [HA] = Ka/ [H+] = {1.0x 10 -3] / 1.0 x 10-5 = 100
Since the pH is increased to 5.0 which is greater than its pKa, the acid is more dissociated.
3) The acid is a dicarboxylic acid with pK values, 1.3 and 4.3.
When the aqueous solution of H2Ox has pH = Pk1 = 1.3 , exactly 50% of one -H is replaced that is we have 50% H2Ox and 50% HOx- . similarly at pH = Pk2 = 4.3 we have 50% of HOx- and 50% of Ox=.
Thus at a mid pH of 2.8 = (1.3+4.3)/2 , we have 100% HOx-.
a) The dominant form at pH = 3.3 will be HOx- [since the pH crossed the 100% HOx- pH but not reached 50% Ox-=]
b) the second most dominat form is Ox= since the pH is nearing the pK2.
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