10.3 Acid-Base Stoichiometry

Titration

• A method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration.– You can find the concentration of an acid/base

solution by titrating with a basic/acid solution of known concentration.

– Acid-Base titrations are neutralization reactions

Titration Procedure

1) A measured volume of an acidic or basic solution of unknown concentration is placed in a flask. An acid/base indicator is added.

2) A buret is filled with the titrating solution of known concentration. This is called the standard solution, or titrant.

3) The standard solution is added slowly and mixed into the solution in the beaker until the reaction reaches the end point, when the indicator changes colour.

Titration

Titration

• In a titration, an indicator is used to determine the end point of the reaction.

• At the end point, the indicator will change colour.

• The equivalence point or stoichiometric point occurs when equal moles of hydronium (H3O+) and hydroxide ions (OH-) have been reacted to produce water.

Titration

• The equivalence point is not always equal to the end point and does not always occur at a pH of 7.– pH 7 for strong acid/strong base– pH <7 for strong acid/weak base– pH >7 for weak acid/strong base

• We try to chose an indicator that will change when the equivalence point is reached.

Titration Curve

• In the titration of a strong acid by a strong base, a steep rise in the pH of the acid solution indicates that the acid has been neutralized by the base.

• The point at which the curve flexes is the equivalence point of the titration.

Titration

• Reminder – the point is to find the unknown concentration of an acid or base!!!

• At the end of the day, this is stoichiometry.

• When you perform a titration you want to make sure you record the volumes you started with and the final volume of your titrant.

Examples

• What is the molarity of a nitric acid solution if 43.33 mL of 0.1000 M KOH solution is needed to neutralize 20.00 mL of the acid solution?

• What is the concentration of a household ammonia cleaning solution if 49.90 mL of 0.5900 M HCl is required to neutralize 25.00 mL of the solution?

Sample Problem

• Several 10.00 mL samples of sulfuric acid solution of unknown concentration are titrated with a 0.100 mol/L solution of sodium hydroxide. (the burette contains the sodium hydroxide solution.) The endpoint was determined using phenolphthalein indicator.

Trial 1 2 3

V1 (mL) 00.10 12.52 25.10

V2 (mL) 12.52 24.98 37.62

ΔV (mL) 12.42 12.46 12.52

Homework

ReadSection 10.3

Questionsp. 484 # 1-3p. 485 # 3