Acid

1
Acid Base Equilibria

• Buffer Solutions
• Question Was the ICE Problem set up needed?
• Answer No. The assumption of x ltlt HA, A- is
  valid for all traditional buffers
• Traditional Buffer
• Weak acid (3 lt pKa lt 11)
• Ratio of weak acid to conjugate base in range 0.1
  to 10
• mM concentration range

2
Acid Base Equilibria

• Buffer Solutions
• Since ICE not needed, can just use Ka equation
• Ka HA-/HA HA-o/HAo
• (always valid) (valid for traditional
  buffer)
• But log version more common
• pH pKa log(A-/HA)
• Also known as Henderson-Hasselbalch Equation

3
Acid Base Equilibria

• Buffer Solutions
• Ways to make buffer solution
• Mix weak acid and conjugate base
• Add strong base to weak acid (weak acid must be
  in excess) this converts some of the weak acid
  to its conjugate base
• Add strong acid to weak base (weak base must be
  in excess) this converts some of weak base to
  its conjugate acid

4
Acid Base Equilibria

• Example Problems
• How many moles of hydroxyl ammonium chloride
  (HONH3Cl-) needs to be added to 500 mL of 0.020
  M HONH2 to obtain a buffer solution with a pH of
  6.20? The pKa for HONH3 is 5.96.
• What is the pH of a solution made from mixing 400
  mL of 0.018 M CH3CO2H (pKa 4.75) with 100 mL of
  0.024 M NaOH?

5
Acid Base Equilibria

• Last Example Problem
• How many mL of 0.0500 M NaOH should be added to
  50.0 mL of 0.00850 M methyl ammonium chloride
  (CH3NH3Cl-) in order to make a buffer with a pH
  of 10.80? The pKa for CH3NH3 is 10.645.

6
Acid Base Equilibria

• Additional Questions
• Which of the following will result in a
  traditional buffer?
• 0.00100 M HNO3 0.00200 M NaNO3
• 0.010 M NH4Cl 0.003 M NH3
• 1.0 x 10-5 M CH3CO2H 0.010 M NaCH3CO2
• 0.0020 M HCl 0.010 M CH3CO2H
• 0.0020 M HCl 0.010 M NaCH3CO2