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Volumetric Analysis (Titration)

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What is it? The concentration of solutions of acids and bases can be determined accurately by a technique called volumetric analysis. This involves reacting the solution of unknown concentration with a solution of accurately known concentration (a standard solution).

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Strong acid v weak base This time phenolphthalein would be completely useless. However, methyl orange starts to change from yellow towards orange very close to the equivalence point. You have to choose an indicator which changes colour on the steep bit of the curve. If the solution becomes red, you are getting further from the equivalence point

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Primary Standards Substances that are so pure that the amount of substance, in mole, can be calculated accurately from their mass are called primary standards. To be useful, a primary standard should: be readily obtainable in a pure form have a known formula be easy to store without deteriorating or reacting with the atmosphere. It is also desirable that the substance be inexpensive and that it have a high molar mass in order to minimise the effect of errors in weighing.

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EQUIPMENT Volumetric Analysis Equipment is measured to 4 decimal places – any you calculate should not exceed 4 decimal places as you would be claiming an accuracy that you do not know. Burette – wash it with what you are filling it with Conical flask – rinse with what you are filling it with Volumetric Flask – rinse with water Pipette – rinse with what you are pipetting (use a separate pipette for each solution)

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Sources of Error Titration is a very accurate procedure in measuring volumes of liquids, however, there are many sources of error that can account for variance in results. 1.Volumetric equipment – calibration errors so incorrect measurements 2.Misjudging colour change of indicator 3.Incorrect reading of volumes/scales 4.Contaminated solutions (using same pipettes, not washing with correct solutions) Water is a very common contaminant as if it is in equipment it will dilute solutions and cause errors in results

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Titration Calculations If the question asks for you to take an aliquot (a measured volume of solution, usually delivered by a pipette), be aware that you will need to multiply the number of mol by a dilution factor Eg. 20ml aliquot from a 250ml solution = 20/250

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Example 1 A standard potassium carbonate solution is made by adding 1.227 g of K 2 CO 3 to a 250 mL volumetric fl ask and filled to the mark with water. 20.00 mL aliquots are taken and titrated against sulfuric acid, H 2 SO 4 using methyl orange indicator. The average titre was 22.56 mL of sulfuric acid. a Write the equation for the reaction. b Calculate the concentration of the K 2 CO 3 solution. c Calculate the concentration of the sulfuric acid solution.

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Write the equation for the reaction. K 2 CO 3 + H 2 SO 4 K 2 SO 4 + H 2 O + CO 2 K 2 CO 3 250ml 1.227g 20ml aliquot 22.56ml H 2 SO 4 20ml K 2 CO 3

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n(K 2 CO 3 ) = m/M = 1.227/138.2 = 0.008878 mol c(K 2 CO 3 ) = n/V = 0.008878/0.25 = 0.03551 M Calculate the concentration of K 2 CO 3 K 2 CO 3 250ml 1.227g 20ml aliquot 22.56 ml H 2 SO 4 20ml K 2 CO 3

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n(K 2 CO 3 ) aliquot = n(K 2 CO 3 ) x dilution factor = 0.008878 x (20/250) = 0.0007102 mol ***Stoichiometry Table*** n(H 2 SO 4 ) = n(K 2 SO 4 ) = 0.0007102 mol C (H 2 SO 4 ) = n/v = 0.0007102/0.02256 = 0.01348M Calculate the concentration of H 2 SO 4 K 2 CO 3 250ml 1.227g 20ml aliquot 22.56 ml H 2 SO 4 20ml K 2 CO 3

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