NJC Planning ans

National Junior College SH1 H2 Chemistry 65 Planning Experiments [Planning]: Titration 1 – Acid-Base 1 Citrus fruits such as limes, lemons and oranges are an excellent source of vitamins and they also contain a mixture of weak acids such as citric acid, tartartic acid and ascorbic acid. You are required to plan an experiment involving titration that will enable you to determine an accurate value for the concentration of H+ present in lemon juice. (Assume that the acid present in lemon juice is a monobasic acid.) You are provided with:  FA 1 100 cm3 of fresh (filtered) lemon juice  FA 2 400 cm3 of 0.150 mol dm–3 aqueous sodium hydroxide  Phenolphthalein and methyl orange indicator  Deionized water  Common laboratory apparatus (a) It is found that 1 drop of FA 1 required 5 drops of FA 2 for complete reaction. Explain why the given FA 1 should not be titrated with FA 2 directly. The given FA 1 is too concentrated. 25.0 cm3 of FA 1 will require 125 cm3 of FA 2 (525.0 1251 cm3) for a complete neutralisation, which exceeds the capacity of the burette. (b) State the dilution factor for the FA 1 solution so that 25 cm3 of this diluted solution would require approximately 25 cm3 of FA 2 for complete reaction. Since H+ and OH– reacts in 1:1 mole ratio, FA 1 needs to be diluted to ~ 0.150 mol dm–3. Dilution factor = 5 1 = 5 (c) Outline, step by step, how you would prepare 250 cm3 of diluted solution of FA 1 based on your answer in part (b). Vol. of FA 1 required to dilute to 250 cm3 = 250 5 = 50.00 cm3 Step 1: Transfer 50.00 cm3 of FA 1 using a burette into a 250 cm3 graduated flask. Step 2: Add deionized water to the graduated flask and top it up to the graduated mark Step 3: Stopper the flask, invert and shake well to obtain a homogeneous solution and label the diluted FA 1 as FA 3

National Junior College SH1 H2 Chemistry 66 (d) Give a detailed sequence of steps to produce a set of consistent titres that could be used to find the concentration of H+ present in lemon juice. In your plan, you should include  details of apparatus and quantities of reagent to be used;  the essential details of the titration procedure;  the appropriate data to be recorded . Procedure: 1. Top up a 50.00 cm3 burette with FA 2. Record the initial burette reading. 2. Transfer 25.0 cm3 of FA 3 into a conical flask using a 25.0 cm3 pipette and add 2 - 3 drops of phenolphthalein. 3. Titrate it against FA 2 until the solution turns from colourless to pale pink. Record the final burette reading and calculate volume of FA2 used . 4. Repeat the titration until results are consistent within 0.10 cm3. Titration Rough 1 2 Final burette reading / cm3 Initial burette reading / cm3 Vol. of FA 2