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 used / cm3 (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. Amt of H+ in 25.0 cm3 of diluted lemon juice, FA 3 = Amt of NaOH reacted =  40.150 1.50 101000 y y  mol Amt of H+ in 250 cm3 of FA 3 =  42501.50 1025.0 y  =  31.50 10ymol Amt of H+ in 50.00 cm3 of FA 1 =  31.50 10ymol Conc. of H+ in FA 1 =  310001.50 1050.00 y  = 23.00 10ymol

National Junior College SH1 H2 Chemistry 67 [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. (a) Preliminary consideration: Dependent variable in this expt is volume of NaOH(aq) required. For the same no. of moles of acid to reach end-point: For monobasic acid: Same no. of moles of NaOH(aq) required since: HA(aq) + NaOH(aq)  NaA (aq) + H2O (l) For dibasic acid: TWICE no. of moles of NaOH(aq) required since H2A(aq) + 2 NaOH(aq)  Na2A (aq) + 2H2O (l) Suitable volumes of 0.010 mol dm3 NaOH(aq) used are 20.00 cm3 (to neutralise monobasic acid) and 40.00 cm3 (to neutralise dibasic acid). Note: 1. To choose an appropriate minimum volume of NaOH(aq) used for titration, the percentage error/uncertainty due to apparatus measurement should not exceed 0.5%. Percentage error = uncertainty×no. of reading taken100%quantity measured  (< 0.5%) 2. To choose an appropriate maximum volume of NaOH(aq) used for titration, the max. volume should be kept to 80% of the capacity of the measuring apparatus.

National Junior College SH1 H2 Chemistry 68 To determine the mass of acid used to prepare standard solution of acid using 250 cm3 graduated flask: Amt of NaOH(aq) in 20.00 cm3 = 0.02 × 0.01 = 2.0 × 10-4 mol Amt of acid present in 25.0 cm3 = 2.0 × 10-4 mol (Since H+ OH– for monobasic acid) Amt of acid present in 250 cm3 graduated flask = 2.0 × 10-3 mol Mass of acid needed = 2.0 x 10-3 × 90 = 0.18 g Phenolphthalein is used as an indicator. At equivalence point, pH >7 (a weak acid-strong base titration), since organic acids are weak acids. Procedure: 1. Weigh accurately about 0.18 g of the organic acid solid in a dry and clean weighing bottle using an electronic balance. 2. Transfer the organic acid solid into a small beaker and re-weigh the residual solid in the weighing bottle. Record the actual mass transferred to the beaker. Mass of weighing bottle and organic acid solid / g Mass of weighing bottle and residual organic acid solid / g Mass of organic acid solid used / g 3. Add deionised water into the small beaker to dissolve the organic acid solid and transfer all solution and washings into a 250 cm3 graduated flask. Make up to mark with deionised water. Stopper the flask, invert and shake thoroughly to obtain a homogenous solution. Label as FA 2. 4. Pipette 25.0 cm3 of FA 2 into a conical flask and add 2 - 3 drops of phenolphthalein to the solution. 5. Top up a 50.00 cm3 burette with NaOH. Record the initial burette reading. Titrate against the 0.010 mol dm3 of NaOH(aq) until the solution turns from colourless to pale pink. Record the final burette reading and calculate volume of NaOH used. 6. Repeat the titration until results are consistent (two titres within ±0.10 cm3). (b) Interpretation of data to determine basicity of the organic acid  If volume of NaOH(aq) used for the titration is 20.00 cm3, organic acid is monobasic.  If volume of NaOH(aq) used for the titration is 40.00 cm3, organic acid is dibasic.

National Junior College SH1 H2 Chemistry 69 [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) +