ACID BASE TITRATION OBJECTIVES 1. To demonstrate the basic laboratory technique of titration 2. To learn to calculate molarity based on titrations INTRODUCTION Molarity (M) or molar concentration is a common unit for expressing the concentration of solutions. It is defined as the number of moles of solute per liter of solution (or millimoles of solute per milliliter of solution). The concentration of a basic solution can be determined by titrating it with a volume of a standard acid solution (of known concentration) required to neutralize it. The purpose of the titration is the detection of the equivalence point, the point at which chemically equivalent amounts of the reactants have been mixed. The amount of reactants that have been mixed at the equivalence point depends on the stoichiometry of the reaction. In the neutralization reaction of HCl and NaOH, the equivalence point occurs when one mole of HCl reacts with one mole of NaOH. However, in the reaction of H3PO4 and NaOH, the equivalence point occurs when one mole of H3PO4 reacts with 3 moles of NaOH. HCl + NaOH -----> NaCl + H2O H3PO4 + NaOH -----> Na3PO4 + 3 H2O (1) (2)
For example, the titration of 16.00 mL of 0.184 M HCl requires 25.00 mL of a NaOH solution. To find the molarity (molar concentration) of the NaOH solution: 0.01600 L HCl x 0.184 moles HCl = 0.00294 moles HCl 1 L solution (3)
0.00294 mol HCl x 1 mole NaOH = 0.00294 moles NaOH 1 mole HCl 0.00294 moles NaOH = 0.118 M NaOH 0.02500 L NaOH (5)
To determine when neutralization occurs, an indicator such as phenolphthalein can be used. An indicator is a substance which undergoes a distinct color change at or near the equivalence point. The point at which the indicator changes color and the titration is stopped is called the endpoint. Ideally, the endpoint should coincide with the equivalence point. Phenolphthalein is colorless in acidic solution and reddish violet in basic solution.
PROCEDURE In this experiment, you will...