SASS Chem P3 Volumetric Analysis Revision

Page 1 ST ANDREW’S SECONDARY SCHOOL NAME TEACHING GROUP Chemistry 6092 Practical Theory Practice Volumetric Analysis 1 White vinegar is a colourless solution containing ethanoic acid. You are provided with two samples of white vinegar labelled A and B. You are going to investigate the reaction between these samples and 0.400 mol/dm3 aqueous sodium hydroxide. Instructions You are going to do four titration experiments. Rinse and fill a burette with A. (a) Experiment 1 • Use a volumetric pipette to add 25.0 cm3 of 0.400 mol/dm3 aqueous sodium hydroxide to a conical flask. • Add five drops of thymolphthalein indicator to the conical flask. • Record the initial burette reading in Table 1.1. • Add A from the burette white swirling the flask, adding drop by drop near the end-point, until the solution changes colour. • Record the final burette reading in Table 1.1. Experiment 2 • Empty the conical flask and rinse it with distilled water. • Refill the burette if necessary. • Repeat Experiment 1. Complete Table 1.1 with the volume used in each experiment. Table 1.1 Experiment 1 Experiment 2 Final burette reading/ cm3 21.40 42.90 Initial burette reading/ cm3 0.00 21.40 Volume of A used/ cm3 (b) Experiments 3 and 4 • Pour away any A remaining in the burette. • Rinse and fill the burette with B. • Empty the conical flask and rinse it with distilled water. • Repeat Experiments 1 and 2 using B. • Complete Table 1.2 for Experiments 3 and 4. Table 1.2 Experiment 3 Experiment 4 Final burette reading/ cm3 33.10 42.90 Initial burette reading/ cm3 0.00 10.00 Volume of B used/ cm3

Page 2 (c) Calculate the mean volume of A and B needed to neutralise 25.0 cm3 of 0.400 mol/dm3 aqueous sodium hydroxide. [2] (d) Suggest why the titrations using A and B are repeated. ..................................................................................................................................................... [1] (e) The equation for the reaction between ethanoic acid and sodium hydroxide is shown. CH3COOH + NaOH ® CH3COONa + H2O The answer to (c) shows the mean volume of B used to neutralise 25.0 cm3 of 0.400 mol/dm3 aqueous sodium hydroxide. Calculate the concentration (in mol/dm3) of ethanoic acid in B. Give your answer to an appropriate number of significant figures. [2] (f) Use your answer to (e) to calculate the mass of ethanoic acid in 500 cm3 of B. [2] [Ar: C, 12; O, 16; H, 1] (g) Use your answers to (c) and (f) to calculate the mass of ethanoic acid in 500 cm3 of A. [1] (h) Suggest why the conical flask should be rinsed with distilled water and not 0.400 mol/dm3 aqueous sodium hydroxide between titrations. ......................................................................................................................................................... .....................