the common ion & buffering. common ion effect the ionization of an acid (or base) is limited by...
TRANSCRIPT
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?