This experiment will test and exercise the principles of acid-base titration and determination of equivalence, as well as exercises on the determination of the pKa of a weak acid. Through the use of the known reactivity of a base, an acid of unknown reactivity and measured mass will be reacted to determine equivalence, and equivalent weight, and by recording changes in pH, these data can be used to determine the pKa. Sodium hydroxide, the base, reacts in a 1:1 mole-to-mole ratio with free protons. By determining the number of moles of free hydrogen protons released by the acid, and measuring the mass of the amount of acid dissolved in the solution, one can calculate the equivalent weight. By recording at which volume of added NaOH the pH changes are the largest, one can determine the equivalence point and extrapolate the pKa. Success will be determined by comparing the resulting pKa with the table of pKa values listed at the back of the text with a reasonable result being within 10% of any listed value.
Experimental
| Reactions:
HA + H2O 
H3O+ + A-
B- + H3O+
H2O + BH
NaOH + HA
H2O + Na+ + A- |
Formulas:
Normal = eq / mol EW = g / eq = (g / mol)(mol / eq) = MW / eq eqacid = eqbase NacidVacid = NbaseVbase pH = -log [H3O+] pKa = -log Ka Kw = KaKb Kw = [H3O+][OH-] pH = pKa + log [A-]/[HA] |
| Symbol | Common Name | Molecular Weight
(g.mol-1) |
| Unknown | Unknown acid | Unknown |
| NaOH | Sodium hydroxide | 39.9969 |
| C20H14O4 | Phenolphthalein | 318.33 |
Materials:
1 50 mL buret
1 250 mL Beaker
1 dropper
1 stir plate
1 pH meter
