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. ......................................................................................................................................................... ......................................................................................................................................................... ..................................................................................................................................................... [2] (i) Some vinegars are brown instead of colourless. Suggest why this titration method would not be suitable for finding the concentration of ethanoic acid in brown vinegar. ......................................................................................................................................................... ..................................................................................................................................................... [1]

Page 3 2 Milk of magnesia is a suspension of insoluble magnesium hydroxide in water. It is taken by people who have stomach pain caused by indigestion and works by neutralising acid in the stomach. P is an aqueous solution of volume 110 cm3 prepared by reacting 10.0 cm3 of milk of magnesia with an excess of hydrochloric acid, HCl. In preparing P, all the magnesium hydroxide in the 10.0 cm3 of suspension reacted when it was added to 100 cm3 of 1.00 mol/dm3 hydrochloric acid, an excess. Mg(OH)2 + 2 HCl ® MgCl2 + 2 H2O You are to determine by titration the amount of acid remaining in P. Q is 0.527 mol/dm3 sodium hydroxide, NaOH. (a) Put P into the burette. Pipette a 25.0 cm3 portion of Q into a conical flask and titrate with P, using an indicator. Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results. Results Titration number 1 2 3 4 5 Final burette reading/ cm3 19.70 41.60 20.80 40.70 19.80 Initial burette reading/ cm3 0.00 19.70 1.00 20.80 0.00 Volume of P used/ cm3 Best titration results (✓) Use the best titration results in the table to calculate the average volume of P used. [1] (b) Q is 0.527 mol/dm3 sodium hydroxide, NaOH. Calculate the number of moles of sodium hydroxide in the volume of Q used. [1] (c) Using the equation shown and your answer to (b), deduce the number of moles of hydrochloric acid that reacted with the volume of Q used. [1] (d) Using your answer to (c) and the average volume of P calculated from the titration results, calculate the number of moles of hydrochloric acid in 110 cm3 of P. [1] (e) Calculate the number of moles of hydrochloric acid in 100 cm3 of 1.00 mol/dm3 hydrochloric acid. [1]

Page 4 (f) Using your answers from (d) and (e), calculate the number of moles of hydrochloric acid that reacted with the magnesium hydroxide in the milk of magnesia. [1] (g) Using your answer to (f), calculate the concentration of magnesium hydroxide in milk of magnesia in g/dm3. [2] [Ar: Mg, 24; O, 16; H, 1] 3 Brass is an alloy of copper used for making screws. You are going to use titration to determine the percentage by mass of copper in a brass screw. The brass screw has been dissolved in nitric acid to form a solution containing aqueous Cu2+ ions. When excess aqueous potassium iodide is added to the aqueous Cu2+ ions, iodine and a white precipitate of copper(I) iodide are produced. 2 Cu2+ (aq) + 4 I– (aq) ® 2 CuI (s) + I2 (aq) The amount of iodine produced is found by titration with aqueous thiosulfate, S2O32–. 2 S2O32– (aq) + I2 (aq) ® 2 I– (aq) + S4O62– (aq) P is 0.120 mol/dm3 aqueous thiosulfate ions. Q contains aqueous Cu2+ ions made from dissolving a 9.45 g brass screw in nitric acid and the solution is made up to 1.00 dm3. R is 0.5 mol/dm3 aqueous potassium iodide. S is starch solution which you will use as an indicator. (a) (i) Instructions • Put P into the burette. • Pipette 25.0 cm3 of Q into a conical flask. • Use a measuring cylinder to add 20 cm3 of R, which is an excess, to Q. A white precipitate forms in a dark brown solution. • Add P from the burette to the flask until the solution turns light brown. Then use a measuring cylinder to add about 1 cm3 of S to the flask. A dark-coloured solution forms. • Continue adding P slowly from the burette until one drop of P causes the dark colour to disappear leaving a white precipitate. • Record your titration results in an appropriate format in the space provided. Repeat the titration as many times as you consider necessary. Results Titration number 1 2 3 4 Final burette reading/ cm3 24.90 48.80 47.50 24.50 Initial burette reading/ cm3 0.00 24.90 22.90 0.00 Volume of P used/ cm3 Best titration results (✓) (ii) Use your titration results to obtain the average volume of P used. [1]

Page 5 (b) (i) Calculate the amount in moles, of thiosulfate ions present in the average volume of P. [1] (ii) Use the equations to determine the amount in moles, of Cu2+ ions in 1.00 dm3 of Q. [2] (iii) Use your answer to (b)(ii) to calculate the mass of c