Buffers Lab

Introduction:

pH:In pure water, the following equilibrium exists:

H2O + H2O OH- + H3O+

where at room temperature (25°C), the molar concentrations ofOH- + H3O+(H+) are 1 x 10-7 mol/L.

pH is calculated as the negative logarithm of the molar concentration of the hydrogen ion. The equation is:

pH = -log10[H+]

pH = -log (1 x 10-7)
pH = -(-7.00)
pH = 7.00 therefore: pure water has a pH of 7.00

Acids:

  • a substance that dissolves in water to produce a solution in which [H+] is greater than 10-7 M.
  • proton donor to some other substance
  • electron pair acceptor

HCl + H2O Cl- + H3O+

(acetic acid) CH3COOH CH3COO- + H3O+

Bases:

  • compound that dissolves in water to give a solution in which [OH-] is greater than 10-7 M.
  • proton acceptor is called a base
  • electron pair donor

(Sodium Hydroxide) NaOH (s) ---> Na+ (aq) +OH- (aq)

(ammonia) NH3 (aq) + H+ (aq) ---> NH4+ (aq)(ammonium ion)

Materials(per student):

  • 10 mL distilled water(pH 7) 15 mL 0.1 mol/L acetic acid (CH3COOH)
  • 5 medium test tubes(ensure they are clean)
  • 1 large test tube0.1 mol/L hydrochloric acid(HCl)
  • test tube rack0.1 mol/L sodium hydroxide(NaOH)
  • pipettes0.1 g sodium acetate(CH3COO-Na+)
  • balance rubber stoppersuniversal pH indicator paper

Procedure:

  1. Arrange 5 clean test tubes in the test tube rack, and label them A, B, C, D, and E.
  2. Measure 15 mL of 0.1 mol/L acetic acid solution into the large test tube. Use the balance to measure 0.10 g sodium acetate. Add the solid to the acetic acid, stopper the top and shake the mixture until it dissolves. This is the buffer solution.
  3. Divide the buffer solution equally placing 5 mL into each of test tubes B, C, and D.
  4. Measure 5 mL of distilled water into each of test tubes A and E.
  5. Measure the pH of each test tube using the universal pH indicator paper.
  6. To each of test tubes A and B, add 5 mL of 0.1 mol/L hydrochloric acid, 1 mL at a time, recording the pH after each addition using the universal pH indicator paper. Record the pH of the 0.1 mol/L hydrochloric acid.
  7. To each of test tubes D and E, add 5 mL of 0.1 mol/L sodium hydroxide, 1 mL at a time, recording the pH after each addition using the universal pH indicator paper. Record the pH of the 0.1 mol/L sodium hydroxide.
  8. To test tube C, add 5 mL of distilled water, 1.0 mL at a time, recording the pH after each addition using the universal pH indicator paper.
  9. Explain the significance of each of the test tubes in establishing the action of a buffer. What do buffers do?

Note:Record the pH of the 0.1 mol/L hydrochloric acid and 0.1 mol/L sodium hydroxide solutions and record in the chart.

Ensure that all stock solutions remain free of contaminants. It may be wise to remove a little of the stock solutions (0.1 mol/L hydrochloric acid, 0.1 mol/L sodium hydroxide and distilled water) to a clean beaker so that contamination does not occur.

Observations:

Test Tube / pH of Solutions / Significance of Test Tube in Establishing the Action of the Buffer
Initial / After 1 mL / After 2 mL / After 3 mL / After 4 mL / After 5 mL
A
B
C
D
E
pH of the 0.1 mol/L hydrochloric acid / pH of the 0.1 mol/L sodium hydroxide.

Discussion:

  1. a) Describe, in detail, how a buffer system, like the one used in this lab, works (a chemistry text book may be required – give the chemical equations for this system when an acid or a base is added).
  2. Describe Le Châtelier’s Principle and show how it pertains to the buffer system used in this lab.

Observation Table:

pH readings of a buffer system after adding small volumes of an acid and a base and its significance.

Test Tube / pH of Solutions / Significance of Test Tube in Establishing the Action of the Buffer
(be specific)
Initial / After 1 mL / After 2 mL / After 3 mL / After 4 mL / After 5 mL
A
B
C
D
E
pH of the 0.1 mol/L hydrochloric acid / pH of the 0.1 mol/L sodium hydroxide.