Main Idea: Titrations are an application of acid-base neutralization reactions that require the use of an indicator. 1 Titrations

Stoichiometry 2 The stoichiometry of an acid-base neutralization reaction is the same as that of any other reaction that occurs in solution (they are double displacement reactions, after all). For example, in the reaction of sodium hydroxide and hydrogen chloride, 1 mol of NaOH neutralizes 1 mol of HCl: NaOH (aq) + HCl (aq)  NaCl (aq) + H 2 O (l) Stoichiometry provides the basis for a procedure called titration, which is used to determine the concentrations of acidic and basic solutions.

Titration 3 Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. If you wish to find the concentration of an acid solution, you would titrate the acid solution with a solution of a base of known concentration. You could also titrate a base of unknown concentration with an acid of known concentration.

In the titration of an acid by a base, the pH meter measures the pH of the acid solution in the beaker as a solution of a base with a known concentration is added from the burette. In the absence of a digital pH meter an A/B indicator can be used / /blb1703.html

Titration Procedure 5 1) A measured volume of an acidic or basic solution of unknown concentration is placed in a flask. An acid/base indicator is added.(will change colour at a specific pH, see page 478) 2) A burette 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 equivalence point, which is the point at which moles of H + ion from the acid equal moles of OH - ion from the base. (not necessarily a pH=7)

In the titration of a strong acid by a strong base, a steep rise in the pH of the acid solution indicates that all of the H + ions from the acid have been neutralized by the OH - ions of the base. The point at which the curve flexes is the equivalence point of the titration. 6

Strong-Strong Titration 7 Page 478 shows a typical graph of a strong acid (HCl)/strong base(NaOH) titration. The inital pH of the HCl is – As NaOH is added, the acid is neutralized and the solution’s pH increases gradually. – When nearly all of the H + ions from the acid have been used up, the pH increases dramatically with the addition of an exceedingly small volume of NaOH. – This abrupt change in pH occurs at the equivalence point of the titration. – Beyond the equivalence point, the addition of more NaOH again results in the gradual increase in pH.

Indicators and Titration End Point 8 Many indicators used for titrations are weak acids. – Each has its own particular pH or pH ranges over which it changes color.(page 478) The point at which the indicator used in a titration changes color is called the end point of the titration. – It is important to choose an indicator for a titration that will change color at the equivalence point of the titration. – Remember that the role of the indicator is to indicate to you, by means of a color change, that just enough of the titrating solution has been added to neutralize the unknown solution. Equivalence point ≠ End point! – BUT for strong-strong titrations, the pH change is so steep and so large, that the are approximately equal.

Titration with an Indicator 9

What’s the Point of a Titration Again? 10 To find the unknown concentration of an acid or a base. So you perform the actual titration noting the volume you started with and how much volume of the titrant you added and then... Stoichiometry! (Oh no! Not math! Anything but math!)

Titration Calculations: An Example 11 The balanced equation of a titration reaction is the key calculating the unknown molarity. For example, sulfuric acid is titrated with sodium hydroxide according to this equation: H 2 SO 4 (aq) + 2 NaOH (aq)  Na 2 SO 4 (aq) + 2 H 2 O (l) 1) Calculate the moles of NaOH in the standard from the titration data: molarity of the base (C B ) and the volume of the base (V B ). In other words, C B V B = (mol/L)(L) = mol NaOH in standard 2) Apply the mole ratio of NaOH to H 2 SO 4 (2:1). Two moles of NaOH are required to neutralize 1 mol of H 2 SO 4. 3) Calculate the molarity of the acid using moles of acid and V A, the volume of the acid in liters. C A = mol H 2 SO 4 titrated/V A

Sample Calculation 12 Open to page 482 – Sample problem #1

HOMEWORK QUESTIONS 13 1) What is the purpose of a titration? How is it performed (in general)? 2) What is the difference between the equivalence point and the end point of a titration? 3) Read section10.3 4) Questions 1-3 on page 484 5) Prepare lab on page 488 for tomorrow.

MORE HOMEWORK 14 5) What is the molarity of a nitric acid solution if mL of M KOH solution is needed to neutralize mL of the acid solution? 6) What is the concentration of a household ammonia cleaning solution if mL of M HCl is required to neutralize mL of the solution? 7) How many milliliters of M NaOH would neutralize mL of M H 2 SO 4 ?