NJC Planning Practices

National Junior College SH1 H2 Chemistry 63 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 ( (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. (c) Outline, step by step, how you would prepare 250 cm3 of diluted solution of FA 1 based on your answer in part (b).

National Junior College SH1 H2 Chemistry 64 (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 . (e) A student carried out your plan and obtained an average titre value of y cm3. Outline how you would use this result to determine the concentration of H + present in FA 1.

National Junior College SH1 H2 Chemistry 65 [Planning]: Titration 2 – Acid-Base 2(a) Outline a stepwise titration method by which you can determine whether a solid organic acid provided is monobasic or dibasic. You are provided with: • 1.0 g of an anhydrous solid organic acid of Mr of 90 • 0.010 mol dm–3 aqueous sodium hydroxide • deionized water • common laboratory apparatus In your plan, you should • indicate the mass of acid to be used and the indicator; • justify the amount and concentration of the chemicals used and, • show the appropriate recording of data. (b) Explain how you would interpret the data to determine obtain the basicity of the organic acid. 14 ng

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