2 Concentration What equation do we know involving concentration? n = cV Concentration is expressed in mol L-1, which is given the symbol MAnother measure for concentration is mass per unit volume, g L-1, this is found easily by dividing the mass (in g) by the volume (in L).To convert mol L-1 to g L-1 multiple the molarity by the molar mass of the solute
3 Your TurnPage 34Question 11aQuestion 13Question 14b,c
4 Standard SolutionA solution with an accurately known concentration is called a standard solution.You can not simply make a standard solution by dissolving a measured mass of a substance in water to give a known volume of solution. It just doesn’t always work.Only a few chemicals encountered can do this, many chemicals are impure as they react with the atmosphere or decompose
5 Standard SolutionsSubstances that are so pure that the amount of substance, in mole, can be calculated accurately from their mass are called primary standards.A primary standard should:Be readily obtainable in pure formHave a known formulaBe easy to store without deteriorating or reacting with the atmosphereHave a high molecular mass to minimise the effect of errors with weighingBe inexpensive
6 Standard SolutionsPage 29 look at some examples of primary standards.
8 Volumetric AnalysisHelps us to find the amount of a substance in solutionVolumetric analysis involves reacting a measured volume of a standard solution with a measured volume of the solution of unknown concentrationIdeally the solutions are mixed until they have just reacted completely in the mole ratio indicated by the stoichiometric equation.What is this process called????
10 Equivalence pointThe equivalence point is the point during the titration when the solutions have been mixed in the mole ratio shown by the reaction equation.Example2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + H2O(l)The equivalence point is reached when the mole ratio n(HCl) : n(Ca(OH)2) = 2:1So n(HCl) = 2n(Ca(OH)2)
11 Equivalence point and end point An indicator must be used in acid-base titrations to detect the equivalence point.The change in colour during a titration is the end pointSo you need an indicator with an end point which closely matches the equivalence point
12 AccuracyThere are always errors associated with measurements made during experimental work.Typical undertainties associated with volumetric analysis are:20 mL pipette ±0.05 mLBurette ±0.02 mL for each reading250.0 mL volumetric flask ±0.3 mL
13 Accuracy in titrations We always do 3 concordant titres. Why?This is because the volume of a single drop from a burette is about 0.05 mLSo each concordant titre is within a drop of each other.Remember we always use the average titre
14 Your TurnPage 34Question 23For homeworkQuestion 16 and 21