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

National Junior College SH1 H2 Chemistry 66 [Planning]: Titration 3 – Redox Reaction 3 You are to devise a method to determine the percentage purity of potassium nitrite in an impure sample Y. The percentage purity of KNO2 in the given sample Y is around 95% – 98%. Nitrite ions, NO2−, are oxidised to NO3− by acidified potassium manganate(VII), KMnO4. However, nitrite ions decompose in contact with dilute acids. NO2 −(aq) + H+(aq) → HNO2(aq) 3HNO2(aq) → H2O(l) + HNO3(aq) + 2NO(g) You are provided with: • solid sample Y • 0.20 mol dm−3 potassium manganate(VII) • 1.0 mol dm−3 sulfuric acid (a) Bulk solution of Y is prepared by dissolving 1 g of Y in deionised water to form 250 cm3 of solution. Using calculations, explain why the 0.20 mol dm−3 KMnO4 solution provided is not suitable for the titration in this case. [Hint: % purity of KNO2 in the given sample Y is around 95% – 98%] (b) Give an appropriate sequence of steps which enable you to use titration results to determine the percentage purity of KNO2. Your plan should include • suitable starting quantities of KMnO4 and H2SO4 to carry out the above titration by considering the answer in part (a). • steps to prepare solution Y mentioned in part (a). • steps taken to prevent significant decomposition of NO2− ion taking place during titration. You may assume that the rate of redox reaction is faster than rate of decomposition of NO2− ion caused by acid. • the endpoint colour change by considering the colour of the ions in reactants and products. (c) Show, step by step, how you will calculate the percentage purity of potassium nitrite in sample Y. Assume that the titration result is 22.50 cm3.

National Junior College SH1 H2 Chemistry 67 [Planning]: Titration 4 – Iodine-thiosulfate 4 Iodine is an essential element in the human body. It is normally obtained through the diet but in regions of many countries there is an iodine deficiency. Sodium chloride, NaCl, used in food industry can have a very small amount of potassium iodide, KI, added as a source of iodine. The product is sold as ‘iodised salt’. In the manufacture of ‘iodised salt’, sodium chloride and potassium iodide are dry mixed to give a mixture containing 2000 parts per million of potassium iodide. In a manufacturing of ’iodised salt’, too much potassium iodide was accidentally added to the sodium chloride. You are required to outline a plan involving titration to determine the percentage by mass of potassium iodide present in the ‘iodised salt’ sample. You are provided with: • 0.100 mol dm−3 copper sulfate • 0.100 mol dm−3 of thiosulfate ions • starch solution • ‘iodised salt’ • usual apparatus for titration • an electronic balance In your plan, you should include • masses and volumes of reagents to be used from the preliminary consideration (You may assume 20 g of iodised salt can easily dissolve in deionised water to form 25.0 cm3 of solution.) • reagents that must be in excess • how the mass of potassium iodide is to be calculated from the titre obtained by working with arbitrary values like A cm3 24sett

National Junior College SH1 H2 Chemistry 26 [Planning] [Planning] : Gas Collection 1 When the nitrates of the Group 1 metals (Li, Na, K, Rb, Cs) are heated they decompose in one of the following ways. 2MNO3(s) → 2MNO2(s) + O2(g) 4MNO3(s) → 2M2O(s) + 4NO2(g) + O2(g) M represents a Group 1 metal. You are to design an experiment in which the oxygen gas evolved from the thermal decomposition of a known quantity of potassium nitrate is collected and its volume measured. Nitrogen dioxide, NO2, is soluble in water. Oxygen gas, O2, can be regarded as insoluble in water. From the data collected in the experiment, you should be able to determine which of the above equations correctly represents the decomposition of potassium nitrate. (a) Draw a labelled diagram of the apparatus you would use to ▪ decompose the potassium nitrate by heating, ▪ remove any nitrogen dioxide formed, ▪ collect the oxygen gas evolved, and ▪ measure the volume of the gas collected. Your diagram should show how standard pieces of laboratory apparatus are to be assembled. You should show clearly how these pieces of apparatus are connected together. (b) What is the maximum volume of oxygen gas that can be collected in the apparatus shown in your diagram? hard glass test tube 100 cm3 measuring cylinder KNO3

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