Equilibria: Acids and Bases
The Most Important Equilibrium on the Planet. Part 1
A.
B.
What is the pH of the distilled water that you use in lab?
6.0
Each group has an Erlenmeyer flask with water and an indicator. Add a small piece
(~size of a nickel) of dry ice to the water and swirl. Given that the color change for
bromothymol blue indicator occurs between pH = 6.0 to 7.6, what do you estimate the
pH of the solution is after the addition of dry ice?
5.5
C.
Write down the net reaction of CO2(aq) with water.

CO2(aq) + H2O(l)
H2CO3(aq)
Look at the product of this reaction. Could it be an acid or a base?
Write an equation to show its acid/base reaction.

H2CO3(aq) + H2O(l)
HCO3- (aq) + H3O+
(Ignore second deprotonation of carbonic acid Ka2 = 10-11 << Ka = 4.2 x 10-7
How can these two reactions above be used to corroborate your answer to part B?

CO2(aq) + 2H2O(l)
D.
HCO3- (aq) + H3O+
Now add 2 pipets of 1.0 M NaOH to the flask, dropwise.
How has the pH of the solution changed qualitatively?
It has increased
What has happened to the hydronium ion concentration?
[H3O+] has decreased
Write down the reaction of the H3O+ with OH-.
H3O+
+ OH-

H2O
E.
If you added a 1 g chunk of CO2 (s) to 100 mL of water, what is the pH of the solution?
The pKa1 of carbonic acid is 7.88. (hint: This is a weak acid pH problem and is just a
disguised ICE equilibrium problem. Look at the question this way: if initially 1g of CO2
is dissolved in 100 mL of water, what is the equilibrium concentration of products?)
How good was your estimate in step B?
USE: H2CO3(aq) + H2O(l)

HCO3- (aq) + H3O+
Find the initial moles of CO2 = H2CO3 (ASSUMMPTION!!)
1g/44 g mol-1 = 0.023 mol CO2 = 0.023 mol H2CO3
0.023 mol CO2 / 0.100L = 0.0023 M H2CO3
I)
C)
E)
[H2CO3]
0.23
-x
(0.23 – x)
----
[HCO3-]
[H3O+]
0
1x10-7
+x
+x
x
(1x10-7+ x) ~ x
Assume 10-7 is negligible compared to x
Ka = 10(-7.88) = 1.33 x 10-8
1.33 x 10-8 = (x2)/(0.23-x) (assume x is negligible compared to 0.023)
x = 5.5x10-5 = [H3O+] (assumption OK)
pH = -log [2.76x10-5] = 4.25
F.
It is possible for distilled water in equilibrium with CO2 to have neutral pH? Why or why
not?
NO!!! Distilled water in equilbirum with carbon dioxide is always slightly acidic, as we
calculated, about pH 5.5-6.
G.
CO2 is an example of an acid anhydride, which means “an acid without water.” The
Bronsted acid is formed by adding 1 molecule of H2O to the acid anhydride. (You did
this in step C in Part 1.) Other acid anhydrides from automobile exhaust that are major
contributors to acid rain are NO2 and SO3. What common strong acid is SO3 related to?
(hint: Show reaction with H2O.)
SO3 + H2O
 H2SO4