The Common Ion & Buffering

Common Ion Effect

• the ionization of an acid (or base) is limited by the presence of a significant amount of its conjugate base (or acid)

HA + H2O <---> A- + H3O+

• Why?

Common Ion Problem

1. What is the pH of 0.10 M solution of HF? (Ka = 7.2 x10-4)

2. What is the pH of a solution that is 0.10 M HF and also contains 0.15 M F-?

BUFFERS maintain pH!

• A buffer is a solution with a __________ pH, adding acid or base to a buffer doesn’t significantly affect the pH

• Requirements:1) Need an acid to absorb added _____ and a

base to absorb added ____

2) The acid and base ________________ with each other. (so conjugate pairs are used)

Suitable Buffers

• An ideal buffer is prepared from ______ amounts of a weak acid and its conjugate _______

• The pH of a buffer ______ depends on the Ka and can be raised or lowered by altering the ratio of acid/base present

• The action of a buffer is special case of the common ion effect, and allows for a shortcut!!!!!

Equilibrium Expression

• Weak Acid: Ka =

• Solve the expression for [H3O+]

Henderson-Hasselbalch Equation

Weak acid and conj. base

Common Ion Problem Again

Use the H-H Equation to solve this problem again.

1. What is the pH of a solution that is 0.10 M HF and also contains 0.15 M F-? (Ka = 7.2 x10-4)

Henderson-Hasselbalch Equation

• Recall that _______ buffers have equal amounts of acid and conj. base.

• If the Ka of acetic acid is 1.8E-5, what is the pH if a buffer is prepared having equal concentrations of acetic acid and acetate?

Preparing Buffer Solution

• The desired pH must be close to the pKa of the acid

• Increase the pH by adding more _______ and decrease the pH by adding more _______

H-H Equation

Problem 1• Which acid/conjugate base pair would be most

suitable to buffer a pH of 7.5?

Weak Acid Ka

• Acetic acid 1.8 X 10-5

• Dihydrogen phosphate 6.2 x10-8

• Hydrogen phosphate 3.6 x10-13

H-H Equation

Problem 1 cont.

• What is the ratio of conjugate base to weak acid needed to have a pH of 7.5?

H-H Equation

Problem 2• A buffer solution contains 0.20 M acetic acid

and 0.50 M acetate. What is the pH?

Use the H-H Equation

• pH of Buffers• pH of solutions with Common Ions

(saves you the ICE table)• Acid/Base Titrations

Practice Problem

Problem 2 continued.

• Suppose you have 500 mL of the acetic acid/acetate buffer system. What is the pH after you add 25.0 mL of 1.0 M HCl to the buffer?