Honors Chemistry
May 28th
Do Now:
• Identify the conjugate acid or base of the
following, write the Ka or Kb for the forward
direction:
• Cl• BO33• HI
• HBr
Agenda
• LT: I distinguish between a strong and weak acid
or base and calculate Ka’s, and Kb’s
•
•
•
•
•
Do Now
Agenda
Kb
Practice
Titrations
• HW: Ka Worksheet
How to tell if a base is strong or not…..
• We can use an equilibrium constant to figure
this out.
• We call it a Kb it is the base disassociation
constant.
NH3 (aq)  H 2O(l )


NH4 (aq)  OH (aq)
• For the above reaction we need can write an
equilibrium constant

NH3 (aq)  H 2O(l )

NH4 (aq)  OH (aq)

[ NH 4 ][OH  ]
K eq 
[ NH 3 ][ H 2O]


[ NH 4 ][OH ]
K b [ H 2O]K eq 
[ NH 3 ]
• What we can say is that the higher the Kb the stronger
the base and the lower the Kb the weaker the base.
• Strong bases have a Kb greater than 1, weak base have
a Kb that is less than 1.
Example
• What is the pH of a 0.20 M solution of
pyridine, C5H5N, in aqueous solution? The Kb
for pyridine is 1.4 x 10-9.
A Good Rule of Thumb with acid or
base ICE problems
– It can be shown that if the acid concentration, Ca,
divided by the Ka exceeds 100, that is,
if
Ca
Ka
 100
– then this simplifying assumption of ignoring the
subtracted x gives an acceptable error of less
than 5%.
Another important thing with Ka and
Kb
Ka  Kb  K w
You do it
1. A student prepares a 0.45 M solution of a
monoprotic weak acid and determines the pH to
be 3.68. What is the Ka of this weak acid
2. The pH of a 0.40 M solution hypobromous
(HOBr) acid is 5.05. Calculate Ka
3. The Ka for HCN = 6.31 x 10-10. Calculate the
equilibrium concentrations of the hydronium ion
and cyanide ion from a 0.020 M solution of the
acid.
4. A 0.350 M Solution of the weak acid HA has a pH of
1.620. Find the Ka of HA.
5. The [H3O+] of a 0.100 M methanoic acid (HCOOH)
solution is 4.2 x 10-3 M. Calculate the Ka of the acid.
6. What is the Ka for hydrofluoric acid if 0.200 M of
solution produces a concentration hydronium ions
equal to 6.8 x 10-4M?
7. A 0.100 M solution of sodium carbonate has
a pH of 11.618. What is Kb for the carbonate
ion?
8. Calculate the pH of a 1.00 M solution of
hydrofluoric acid (HF). Ka = 3.50 x 10-4.
Repeat the calculation for a 1.00 x 10-2 M
solution.
9. How many grams of chlorous acid must be
dissolved in water to make 100 mL of a HClO2
solution whose pH is 2.40? Ka for HClO2 = 1.1
x 10-2. Molar mass of HClO2 = 68.5 g/mol.
10.A 0.20 M solution of the hypothetical weak
acid HZ is found to have a pH of exactly 3.0.
What is the Ka of the acid HZ?