1
Chemistry 265
November Quiz
2012
Name:
1.
Citric acid is a triprotic acid with pKa1 =3.128 ,p Ka2 = 4.761 and p Ka3 = 6.396.
[3]
(a)
For what pH regions can you prepare buffers using citric acid.
2.128 -4.128, 3.761-5.761 and 5.396-7.396 ie 2.128-7.396
[6]
(b)
Sketch a titration of 10 mL of 0.1 M trisodium citrate with 0.1 M HCl. Show
how you would use the curve to determine the pKas. Mark in each pKa.
2
[2]
(c)
Do you actually expect to see 3 distinct steps? Why or why not?
No
The pKas are rather close, <3
[3]
(d)
Write the equation for each step in the titration.
C3-+ H+ ⇌ HC2HC2- + H+ ⇌ H2CH2C- + H+ ⇌ H3C
[5]
(e)
What is the pH of the 0.1 M trisodium citrate?
C3-+ H2O ⇌ HC2- + OH- Kb = Kw/Ka3
Ka3 = 4.02 x 10-7
Kb = 10-14/4.02 x 10-7 =
[𝑂𝐻 − ]2
𝐾𝑏 =
=
𝐹
2.49 x10−8 ∗ 0.1 = [𝑂𝐻 − ]2
[𝑂𝐻 − ] = 4.99 x 10−5 M
pOH = 4.302 pH = 9.698 =9.7
[5]
(f)
Determine the pH after adding one equivalent of acid.
10 mL added
Total volume now = 20 mL.
C3-+ H+ ⇌ HC2-
3
[HC2-] = 0.05 M
pH =
[5]
(g)
𝑝𝐾𝑎2 +𝑝 𝐾𝑎3 .
2
=
4.761+ 6.396
2
= 5.578
Determine the pH after adding the second equivalent of acid.
20 mL added. Total volume now = 30 mL. F = 0.033
pH =
𝐾𝑎1 + 𝐾𝑎2 .
2
=
3.128+4.761
2
= 3.944
4
[5]
(h)
Determine the pH after adding the third equivalent of acid.
Total volume 40 mL. 1mmol/40 =0.025 M
We will now have a solution of citric acid.
quadratic
pH = 2.40
5
2.
[3]
(a)
There are several ways to prepare buffers in the lab. I am interested in an acetate
buffer. The pKa for acetic acid is 4.76.
What is the role of a buffer?
Resists changes in pH
[3]
(b)
If I wanted to prepare an acetate buffer, and I had acetic acid and sodium hydroxide,
how could I do this? Just give the general idea – I do not need exact amounts.
Mix one into the other till required pH is reached. Follow with pH meter
[5]
(c)
If had a 0.200 M solution of sodium acetate and a 0.400 M solution of acetic acid, in
what ratio would I need to mix them to give me a buffer of pH 5.2?
5.2 = 4.76 + log [A-]/[HA]
0.4 = + log [A-]/[HA]
Let ratio be x:1 for volume ratio acetate to acetic
Then
0.4 = log (x. [A-])/[HA]
2.5 =x*.2/.4 so x = 5 so 5:1
6
3.
EDTA titrations
[3]
(a)
What is a chelating agent?
[2]
(b)
Give an example of an auxiliary complexing agent.
ammonia
[3]
(c)
Calculate Kf’ for the titration of
Where log Kf = 13.89
and α4 = 0.30 for EDTA at pH = 10
2.33 x 1013
[2]
(d)
What is the concentration of free Ca2+ in a solution which is 0.200 F in Ca2+ and 0.1 F in
EDTA. It is buffered to pH 11.
0.19 M
[4]
(e)
Name an indicator that is useful for an EDTA titration. Write an equation to show how it
works. Indicate the colour change involved.
Eriochrome Black T
MgIn + EDTA ⇋MgEDTA + In
Red
Blue