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Titrations

Titrations are a method for determining the concentration of a solution by reacting it with a solution of known concentration. This article explains the stoichiometry, procedure, and calculations involved in titrations using an acid-base neutralization reaction.

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Titrations

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

  2. Stoichiometry • 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) + H2O (l) • Stoichiometry provides the basis for a procedure called titration, which is used to determine the concentrations of acidic and basic solutions.

  3. Titration • 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.

  4. 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. http://wps.prenhall.com/wps/media/objects/3312/3392202/blb1703.html

  5. Titration Procedure • A measured volume of an acidic or basic solution of unknownconcentration is placed in a flask. An acid/base indicator is added.(will change colour at a specific pH, see page 478) • A burette is filled with the titrating solution of knownconcentration. This is called the standard solution, or titrant. • 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)

  6. 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.

  7. Strong-Strong Titration • Page 478 shows a typical graph of a strong acid (HCl)/strong base(NaOH) titration. The inital pH of the HCl is 1.00. • 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.

  8. Indicators and Titration End Point • 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.

  9. Titration with an Indicator

  10. What’s the Point of a Titration Again? • 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!)

  11. Titration Calculations: An Example 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: H2SO4 (aq) + 2 NaOH (aq)  Na2SO4 (aq) + 2 H2O (l) • Calculate the moles of NaOH in the standard from the titration data: molarity of the base (CB) and the volume of the base (VB). In other words, CB VB = (mol/L)(L) = mol NaOH in standard • Apply the mole ratio of NaOH to H2SO4(2:1). Two moles of NaOH are required to neutralize 1 mol of H2SO4. • Calculate the molarity of the acid using moles of acid and VA, the volume of the acid in liters. CA = mol H2SO4 titrated/VA

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

  13. HOMEWORK QUESTIONS • What is the purpose of a titration? How is it performed (in general)? • What is the difference between the equivalence point and the end point of a titration? • Read section10.3 • Questions 1-3 on page 484 • Prepare lab 10.3.2 on page 488 for tomorrow.

  14. MORE HOMEWORK 5) 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? 6) 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? 7) How many milliliters of 0.500 M NaOH would neutralize 25.00 mL of 0.100 M H2SO4?

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