CHE 1316 Laboratory Measurements & Techniques LECTURE 1 Course Introduction Intro to Quantitative Analysis Units of Measurement Solutions and Concentrations Stoichiometric Calculations (Review Chaps. 3 & 4) LECTURE 2 Intro to Titrimetric Analysis Analytical Standards Calculations of Titrimetric Analysis (Acids & Bases)

Stoichiometric Calculations Hill, Petrucci, et al., General Chemistry…CHAPTER 3 Stoichiometric Ratio correction: “B” x A + y B → products OR x A + z C → y B mol B = mol A × y mol B x mol A mol = mass/molar mass mol = concentration × volume

Titrimetric Methods Hill, Petrucci, et al., General Chemistry…SECTION 4.6 Titration: Solution (titrant) is added from buret into a titration vessel containing substance being titrated (analyte). Purpose is to determine the volume of titrant required to exactly react with the analyte. Equivalence Point: The point in a titration at which exactly enough titrant has been added to react with all of the analyte. End Point: That point in a titration at which some detectable change occurs signaling that the equivalence point has been reached. Titration Error: Difference between volume of titrant at the end point and the volume at the equivalence point. Et = Vep − Veq

Analytical Standards Titrimetric analyses depend upon a knowledge of the exact amount of either the titrant added or of the substance being titrated Primary Standard: A solid of such purity that a standard solution can be prepared by dissolving the solid in a solvent and diluting to a known volume of solution. High purity. Typically  99.9% Stable in air. No rxns. with H2O, CO2, O2 No waters of hydration. (NiCl2 6H2O) Readily available at modest cost. Reasonable solubility. Typically  0.01 M Large Molar Mass Secondary Standard: Solution must be standardized by titration with/of a primary standard. e.g., Sodium Hydroxide (NaOH): Low Purity Absorbs Water Rapidly. Hygroscopic Reacts with CO2 Low Molar Mass (40 g/mol) Weight (g) / Molar Mass (g/mol) Direct Method: M (mol/L) = Volume (L)

Standard Solution Properties Stable (i.e., concentration constant over time) Reacts rapidly with analyte Reacts completely with analyte Reacts selectively with analyte Reacts with known stoichiometry (i.e., must know balanced chemical eqution)

Volumetric Calculations x A + y B → products mol B = mol A × y mol B x mol A Fundamental Relationship of Titrimetric Analysis: applies only at the equivalence point of the titration (titrate soln. of A with soln. of B): mol titrant = mol analyte × stoichiometric ratio (MEMORIZE!!!) Example: Standardization of HCl with Na2CO3: Na2CO3(s) + 2 HCl(aq) → 2 NaCl(aq) + CO2(g) + H2O(l) Dissolve 5.000 g of primary standard grade Na2CO3(s) in 500.0 mL of water. Pipet 25.00 mL of this solution into a flask and titrate with an HCl(aq) solution. 40.00 mL of titrant are required to achieve the end point. Determine the concentration of HCl.

Volumetric Calculations Primary Standard Solution of Na2CO3: 0.09434 M At the Equivalence Point of Titration: 5.000 g / 106.0 g mol−1 0.5000 L CNa2CO3 = mass / molar mass volume = mol titrant = mol analyte × stoichiometric ratio 2 mol HCl mol HCl = mol Na2CO3 × 1 mol Na2CO3 2 mol HCl CHCl × VHCl = CNa2CO3 × VNa2CO3 × 1 mol Na2CO3 2 mol HCl CHCl × 0.04000 L = 0.09434 M × 0.02500 L × 1 mol Na2CO3 CHCl = 0.1179 M

Problem 7, p. 115 Consider the titration of ferrous ammonium sulfate with potassium permanganate which proceeds according to the following reaction: 10 Fe(NH4)2(SO4)2 + 2 KMnO4 + 8 H2SO4 → 2 MnSO4 + 5 Fe2(SO4)3 + 10 (NH 4)2SO4 + K2SO4 + 8 H2O Write an equation describing the quantitative relationship between the concentration of ferrous ammonium sulfate and the concentration of potassium permanganate, including the stoichiometric ratio which applies at the equivalence point of this redox titration. Step 1: Identify the titrant and analyte and arrange them in the form of the equality that is true at the equivalence point of the titration. 2 mol KMnO4 mol KMnO4 = mol Fe(NH4)2(SO4)2 × 10 mol Fe(NH4)2(SO4)2

Problem 7, p. 115 Step 2: Expand the previous equation using the equality Concentration (C) × volume (V) = mol: C(KMnO4) × V(KMnO4) = C(Fe(NH4)2(SO4)2) × V (Fe(NH4)2(SO4)2) × 2/10 Don’t forget the stoichiometric ratio!!! You will typically be dealing with concentration and volume when considering the titrant in your analyses (and in ‘practical’ problems that your are asked to work).

Problem 10, p. 116 Step 1: Step 2: (for NaOH) Write an equation describing the quantitative relationship between the weight of H4C10H12O8N2 and the concentration of NaOH, including the stoichiometric ratio, which applies at the equivalence point of the following acid-base titration: 4 NaOH + H4C10H12O8N2 → Na4C10H12O8N2 + 4 H2O Step 1: mol NaOH = mol H4C10H12O8N2 × 4 mol NaOH 1 mol H4C10H12O8N2 Step 2: (for NaOH) C(NaOH) × V(NaOH) = mol H4C10H12O8N2 × 4/1 How do we include the weight of H4C10H12O8N2?

Problem 10, p. 116 Step 3: Expand “mol H4C10H12O8N2” in the previous equation using the equality: mass divided by molar mass equals moles… (i.e., grams/grams mol−1 = mol) C(NaOH) × V(NaOH) = × 4/1 g H4C10H12O8N2 MW H4C10H12O8N2 PRACTICE SUGGESTED PROBLEMS FOR NEXT TIME

Suggested Problems Problem Set 1: p. 114 of your RED laboratory manual: 1, 3, 4c-g, 5-7, 10-12, 15-17, 19, 21, 23, 25, 26, 28, 30, 32 Note that listing is slightly modified from your syllabus… Answers are given on pp. 154-155…