Wednesday Quiz 6
Chem 265
2012
NAME:
[4]
1.
The measured pH of 0.020 M HCl +0.080 M KCl at 25oC is 1.79. From this
information, calculate the activity coefficient of H+ in this solution.
Antilog of
-1.79 = 1.62 x 10-2 = activity H+
γ [H+] = activity H+ = aH+
γ =1.62 x 10-2/0.02 = 0.81
[1]
2.
Why is the pH of a buffer nearly independent of concentration?
Depends on the ratio of A- to HA
[1]
3.
Why does the buffer capacity increase as the concentration of buffer increases?
The amount added is a smaller fraction of the total. – or
there is more acid to react with added base and vice versa
[4]
4.
Calculate the fraction of association (α) for 2.00 x 10-1 M sodium acetate.
Kb = 5.7 x 10-10.
Want to find [HA].
NaA + H2O ⇋ HA + OH-
[𝐻𝐴][𝑂𝐻−]
Kb = 5.7 x 10-10 =
𝐹𝐴−
So [HA] = 1.067x10-5
α =
1.067 𝑥 10−5
2.00𝑥10−1
=
[𝐻𝐴]2
𝐹𝐴−
=
[𝐻𝐴]2
2.00𝑥10−1
= 5.3 x 10-4 or 5.3 x 10-2 %