Buffer solutions

Calculating the pH of a buffer solution:

This can be calculated using the expression for Ka and the Ka value for weak acid used to make the buffer solution.

The dilution of the acid and base need to be taken into account before plugging the values into the Ka expression.

Example:

Remember: pH = -log [H3O+]

pH, pKa, pKb

Acid base equilibrium expressions

Kw = Ka x Kb

Converting between [H3O+] and pH, Ka and pKa, Kb and pKb

-log[Ka]

H3O+, Ka or Kb ---------------> pH, pKa or pKb

inv-log[pKa]

pH, pKa or pKb ----------------> H3O+ , Ka or Kb

calculating pH of a weak base

What is the pH of a 0.1 molL-1 solution of NH3? Given Pka(NH4+) = 9.25

Ka = inv log - 9.25
Ka = 5.62x 10^-10

Kb = Kw/ Ka
Kb = 1 x 10^-14 / 5.62 x 10^-10
Kb = 1.77 x 10^-5

NH3 + H2O -----> NH4+ + OH-
[NH3] = 0.1
[NH4+] [OH-] = X

Kb = [NH4+][OH-] / [NH3]

Kb = [x][x] / [0.1]

[x] = square root of [1.77 x 10^-5] x [0.1]

[x] = [OH-] = 1.33 x 10^-3

pOH = -log[OH-]

pOH = -log [1.33 x 10^-3]

pOH = 2.876

pH = 14 - 2.876

pH = 11.12

calculating pH of a weak acid

Strong vs weak acid....... dilute vs concentrated acid

Acids and conjugate base pairs

Acids and their conjugate bases and bases and their conjugate acid differ by one H

examples:

acids and conjugate bases.

HCl / Cl-

CH3COOH / CH3COO-

H2SO4 / HSO4-

bases and conjugate acids

NH3 / NH4+

CH3NH2 / CH3NH3+

Acids and bases

Strong acids
Completely dissociate in water.
Are mineral acids (any acid that formula starts with an H)
HNO3
HCl
H2SO4
H3PO4

Weak acids
Partially dissociate in water
Are organic acids that have a COOH group
CH3COOH (ethanoic acid)

Strong bases
Any metal hydroxide
NaOH

Weak bases
NH3
CH3NH2