Exp. 28 Objectives

1
Exp. 28 Objectives

• To operate a pH meter
• To graphically determine a stoichiometric point
• To determine the molar concentration of a weak
  acid solution
• To determine the molar mass of a solid weak acid
• To determine the pKa of a weak acid

2
Exp. 28 Introduction

• Potentiometers
• The pH of a solution is measured with a
  potentiometer called a pH meter.
• pH meters measure the potential difference
  between a reference electrode and a Ag/AgCl
  electrode called a glass electrode. The glass
  electrode is sensitive to changes in H in
  solution.

3
Exp. 28 Introduction

• In acid-base titrations, an indicator is chosen
  based on the pH at which the stoichiometric point
  occurs
• Strong acid-strong base pH 7
• Weak acid-strong base pH gt 7
• Unknown acid - ?
• Therefore, a pH meter is used.

4
Exp. 28 Introduction

• A pH meter follows the same basic function as an
  indicator it tells you when you have reached the
  stoichiometric point.

5
Exp. 28 Introduction

• So why use a pH meter?
• pH meters observe a change in pH as a function of
  base volume, allowing us to observe the strength
  of the weak acid, whereas an indicator simply
  tells us when the stoichiometric point has been
  reached.

6
Exp. 28 Introduction

• With a graph of pH vs. volume of NaOH, you can
  determine the number of mL of NaOH used to get to
  the stoichiometric point, and, knowing the
  concentration of the NaOH, can determine moles of
  NaOH at the stoichiometric point
• moles NaOH (mol) volume (L) x M (mol/L)

7
Exp. 28 Introduction

• For a monoprotic acid, HA, one mole of OH-
  neutralizes one mole of acid
• HA(aq) OH-(aq) ? H2O(l) A-(aq)
• The concentration of the acid can be determined
  using equation 28.4
• concentration of HA (mol/L) mol HA
• L
  HA solution

8
Exp. 28 Introduction

• For a diprotic acid, 2 moles of base neutralizes
  1 mole of acid
• H2X(aq) 2OH-(aq) ? 2H2O(l) X2-(aq)
• Using a titration curve similar to Fig. 28.3 and
  equations 28.2 (moles of NaOH) and 28.3
  (stoichiometric equation), you can determine
  moles of solid acid and, from there, use equation
  28.6 to determine the molecular mass of acid
• molar mass (g/mol) mass of solid acid (g)
• moles of
  solid acid

9
Exp. 28 Introduction

• A weak acid undergoes partial ionization
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• When ½ the moles of the HA is neutralized by the
  NaOH during titration, an equal number of moles
  of A- form, and mol HA mol A-. Or HA
  A-, making Ka H.
• This occurs at the halfway point (Fig 28.4) in
  the titration, which is determined after the
  complete titration curve is generated.

10
Exp. 28 Procedural Notes

• Part A will be done by the stockroom
• Part B obtain 35 mL of unknown acid
• Part B.3. Pipet 10 mL into three 150 mL
  beakers, add 25 mL of DI water
• Part B.4. Self-designed data sheet
• Part C.1. Measure 0.15-0.2 g of unknown solid
  acid. Dissolve with 40 mL DI water.

11
Exp. 28 Report Sheet

• Questions 1, 3
• This is a formal.