Buffer Calculations
A buffer is made by adding 0.300 mol HC2H3O2 and
0.300 mol NaC2H3O2 to enough water to make 1.00L
of solution.
(a)What is the pH of the buffer solution?
(b)Calculate the pH of this solution after
0.020 mol of NaOH is added.
(c) For comparison, calculate the pH that
would result if 0.020 mol of NaOH were
added to 1.00L of pure water.
Buffer Calculations
A buffer is made by adding 0.300 mol HC2H3O2
and 0.300 mol NaC2H3O2 to enough water to
make 1.00L of solution.
(a) What is the pH of the buffer solution?
pH = pKa + log [conj. base] / [acid]
= -log 1.8 x 10-5 + log (0.300 / 0.300)
= 4.74
Buffer Calculations
A buffer is made by adding 0.300 mol HC2H3O2
and 0.300 mol NaC2H3O2 to enough water to
make 1.00L of solution.
(b) Calculate the pH of this solution after 0.020 mol
of NaOH is added.
HC2H3O2 + OH-  C2H3O2- + H2O
0.300 0.020
0.300
-0.020 -0.020 +0.020
0.280
0
0.320
pH = 4.74 + log (0.32 / 0.28) = 4.80
Buffer Calculations
For comparison, calculate the pH that would result
if 0.020 mol of NaOH were added to 1.00L of
pure water.
NaOH is a strong base  [OH-] = [NaOH]
[OH-] = 0.020mol / 1.00 L = 0.020M
pOH = 1.70
pH = 12.30
Buffer Preparation
• what most determines the pH range of buffer
is the pKa of the weak acid
• want to have pKa almost equal to desired pH;
use [A-] / [HA] ratio to adjust
• keep [A-] / [HA] ratio between 1/10 and 10/1
pH = pKa ± 1
• Buffers most effective when [A-] / [HA] ratio
approximately equal to 1
Buffer Preparation
Which acid and its sodium salt would you choose to
make a solution buffered at pH 4.25? For your
choice, calculate the ratio of the base to the acid
required to attain the desired pH.
chlorous acid
pKa = 1.95
nitrous acid
pKa = 3.34
formic acid
pKa = 3.74
hypochlorous acid
pKa = 7.54