CHEM-112
Weak Acids pt 1
(Warmup) What is the pH of these solutions? Both reagents fully dissociate in water.
a) 0.20 M HCl
b) 0.050 M Mg(OH)2
Sulfuric acid (H2SO4) is a strong acid, but only for the loss of the first proton. Write out the
reactions for the loss of the first proton and for the loss of the second proton in sulfuric
acid. What is the equilibrium expression for the loss of the second proton?
CHEM-112
Weak Acids pt 1
We’ll be looking at the following acetic acid dissociation data:
[HC2H3O2] pH of
[H3O+] (equil.) [M] [HC2H3O2]
(init.) [M] solution = [C2H3O2-]
(equil.) [M]
(equil.)
0.20
2.73
(demo)
0.15
2.79
0.10
2.88
0.050
3.03
Ka of
solution
pKa of
solution
In your groups, determine the concentration of hydronium in each of the different solutions.
After that, determine the percent dissociation using the following equation:
% 𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑜𝑛 =
[𝐻3 𝑂+ ]𝑒𝑞
∗ 100%
[𝐻𝐴]𝑖𝑛𝑖𝑡
What trend do you see with the percent dissociation?
Determine the pKa of the solution that you worked with. Similarly to pH, pKa should be
reported to two decimal places. How do your pKa values compare to one another?
Extra Practice. Calculate the pKa for acetic acid solutions that you didn’t initially work
with.