Volumetric Analysis Chapter 3

Concentration What equation do we know involving concentration? n = cV Concentration is expressed in mol L-1, which is given the symbol M Another 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

Your Turn Page 34 Question 11a Question 13 Question 14b,c

Standard Solution A 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

Standard Solutions Substances 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 form Have a known formula Be easy to store without deteriorating or reacting with the atmosphere Have a high molecular mass to minimise the effect of errors with weighing Be inexpensive

Standard Solutions Page 29 look at some examples of primary standards.

Your Turn Page 34 Question 12a

Volumetric Analysis Helps us to find the amount of a substance in solution Volumetric analysis involves reacting a measured volume of a standard solution with a measured volume of the solution of unknown concentration Ideally the solutions are mixed until they have just reacted completely in the mole ratio indicated by the stoichiometric equation. What is this process called????

Volumetric Analysis Turn to page 30 and 31

Equivalence point The equivalence point is the point during the titration when the solutions have been mixed in the mole ratio shown by the reaction equation. Example 2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + H2O(l) The equivalence point is reached when the mole ratio n(HCl) : n(Ca(OH)2) = 2:1 So n(HCl) = 2n(Ca(OH)2)

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 point So you need an indicator with an end point which closely matches the equivalence point

Accuracy There are always errors associated with measurements made during experimental work. Typical undertainties associated with volumetric analysis are: 20 mL pipette ±0.05 mL Burette ±0.02 mL for each reading 250.0 mL volumetric flask ±0.3 mL

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 mL So each concordant titre is within a drop of each other. Remember we always use the average titre

Your Turn Page 34 Question 23 For homework Question 16 and 21