K. Cumsille, 2010 AP CHEMISTRY UNIT 2: REACTIONS IN AQUEOUS SOLUTIONS Concentration, Dilutions, Gravimetric Analysis & Titrations

K. Cumsille, 2010 Objectives  Concentration and Dilution  Gravimetric analysis  Titrations  Acid-base  Redox

K. Cumsille, 2010 Concentration and Dilution  Molarity  The amount of solute in a given amount of solvent or solution.  Also known as molar concentration  Equation and units  Examples

K. Cumsille, 2010 Concentration Examples 1) How many grams of potassium dichromate are required to prepare a 250-mL solution whose concentration is 2.16M? 2) In a biochemical essay, a chemist needs to add g of glucose to a reaction mixture. Calculate the volume in cm 3 of a 2.53 M glucose solution she should use for the addition? 3) What volume, in mL, of a M NaOH solution contains 6.22 g of NaOH?

K. Cumsille, 2010  Dilutions  Purpose of dilutions  Definition: the process of preparing a less concentrated solution from a stock solution  Equation M 1 V 1 = M 2 V 2

K. Cumsille, 2010 Dilution Examples 1) Describe how you would prepare 5.00 x 10 2 mL H 2 SO 4 solution, starting with an 8.61 M stock solution of H 2 SO 4. 2) How would you prepare 2.00 x 10 2 mL of a M NaOH solution with a 5.97 M stock solution?

K. Cumsille, 2010 Gravimetric Analysis  An analytical technique based on the measurement of mass.  Technique is usually applied to the isolation of insoluble solids from metathesis precipitate reactions.  Need to know the chemical formula and mass of the ppt to determine the mass of a particular component or ion of the original sample.  Then can determine the percent composition (by mass) of the component in the original compound.

K. Cumsille, 2010 G.A. Examples 1) A gram sample of an ionic metal chloride compound is dissolved in water and treated with excess of silver(I) nitrate. If g of silver(I) chloride precipitates out, what is the percent by mass of chloride ions present in the original metal chloride sample? 2) A sample of g of ionic halide compound containing bromide is dissolved in water and treated with excess silver(I) nitrate. If the mass of the silver(I) bromide precipitates out is g, what is the percent by mass of bromide ions present in the original compound?

K. Cumsille, 2010 Titrations  Acid-Base  A titration in which a solution of accurately known concentration is added gradually to another solution of unknown concentration, until the reactants have come to completion.  Standardizing the standard solution Titrating the standard solution (such as NaOH) against a known acid concentration. Potassium hydrogen phthalate (KHP) is often used as the acid with the known concentration.

K. Cumsille, 2010  Process:  A known amount of KHP is transferred to a flask and diluted with DI water.  NaOH is carefully added to the dilute KHP until the equivalence point is achieved. Equivalence point – Completion of neutralization of the acid by the base Indication of completion/neutralization  Regardless of the acid or base used in a titration, the total number of protons that have reacted at the equivalence point must be equal tot the total number of moles of hydroxide ions reacted.

K. Cumsille, 2010 Example 1. In a titration experiment, Lora finds that mL of NaOH solution are needed to neutralize g KHP. What is the concentration of the solution? 2. How many mL of a M NaOH solution are needed to neutralize 20.0 mL of a M sulfuric acid solution? 3. How many mL of a 1.28 M sulfuric acid solution are needed to neutralize 60.2 mL of a M KOH solution?

K. Cumsille, 2010 Redox Titrations  Titrations between oxidizing and reducing agents.  Equivalence point is achieved when the reducing agent is completely oxidized by the oxidizing agent.  Requires Indicator  Common oxidizing agents are KMnO 4 and K 2 Cr 2 O 7  Similar calculations to acid- base titrations, but more complex.

K. Cumsille, 2010 Redox Titration Examples 1. A mL volume of M KMnO 4 solution is needed to oxidize mL of a FeSO 4 solution in an acidic medium. What is the concentration of the FeSO 4 solution? 2. How many mL of M HI solution are needed to reduce 22.5 mL of a M KMnO 4 solution according to the following eqn: HI + KMnO 4 + H 2 SO 4  I 2 + MnSO 4 + K 2 SO 4 + H 2 O 5Fe 2+ + MnO H +  Mn Fe H 2 O