RI 2025 VA Planning Tutorial 1 Ans

1 Planning Tutorial 1 : Volumetric Analysis (Suggested Answer) Q2(a) Pre-experiment calculations Concentration of Na2S2O3 in FA 4 = 0.0400 mol dm−3 Amount of Na2S2O3 in 250 cm3 of FA 4 = (250/1000)(0.0400) = 0.0100 mol Mr of of Na2S2O3.5H2O = 248.2 Mass of Na2S2O3.5H2O to use = (0.0100)(248.2) = 2.482 = 2.48 g [1] Procedure to prepare 250 cm 3 of FA 4 1. Using an electronic balance, weigh accurately about 2.48 g of Na2S2O3.5H2O solid in a clean and dry weighing bottle. 2. Transfer the Na2S2O3.5H2O solid from the weighing bottle to a 100 cm 3 beaker quantitatively. Rinse the weighing bottle a few times with deionised water and transfer the washings to the 100 cm3 beaker. 3. Using the wash bottle, add some deionised water to the beaker to dissolve the Na 2S2O3.5H2O solid. Using a glass rod, stir the mixture to ensure that all the solid dissolves, adding more deionised water if necessary. 4. Transfer the solution quantitatively into a 250 cm3 volumetric flask. Rinse the beaker a few times with deionised water and transfer the washings into the volumetric flask. 5. Top up the volumetric flask to the 250 cm3 mark with deionised water, using a dropping pipette to add the deionised water drop-wise when nearing the mark. 6. Stopper the volumetric flask and shake the solution thoroughly to ensure that it is homogeneous. Label this solution as FA 4. [1] weighing + dissolving + quantitative transfer [1] correct use of volumetric flask Q2(b) Pre-experiment calculations Amount of Na 2S2O3 in 250 cm3 of FA 4 = (250/1000)(0.0400) = 0.0100 mol Volume of 0.250 mol dm−3 Na2S2O3 solution needed = 0.0100/0.250 = 0.0400 dm3 = 40.00 cm3 [1] Procedure to prepare 250 cm3 of FA 4 1. Fill a burette with 0.250 mol dm-3 Na2S2O3 solution. Record the initial burette reading. 2. Run from the burette 40.00 cm3 of the Na2S2O3 solution into a 250 cm3 graduated flask. Record the final burette reading. 3. Top up the graduated flask to the 250 cm3 mark with deionised water, using a dropping pipette to add the deionised water drop-wise when nearing the mark. 4. Stopper the graduated flask and shake the solution thoroughly to ensure that it is homogeneous. Label this solution as FA 4. Final burette reading / cm3 40.00 Initial burette reading / cm3 0.00 Volume of 0.250 mol dm-3 Na2S2O3 solution used / cm3 40.00 [1] correct use of burette + table [1] correct use of volumetric flask

2 Q2(c) Pre-experiment calculations Dilution factor from 1.00 mol dm−3 to 0.100 mol dm−3 Na2S2O3 = 10 Dilution factor from 0.100 mol dm−3 to 0.0100 mol dm−3 Na2S2O3 = 10 Dilution factor from 0.0100 mol dm−3 to 0.00100 mol dm−3 Na2S2O3 = 10 Hence, volume of 1.00 mol dm−3 solution to prepare 100 cm3 of 0.100 mol dm−3 Na2S2O3 = 10.0 cm3 Volume of 0.100 mol dm−3 solution to prepare 100 cm3 of 0.0100 mol dm−3 Na2S2O3 = 10.0 cm3 Volume of 0.0100 mol dm−3 solution to prepare 100 cm3 of 0.00100 mol dm−3 Na2S2O3 = 10.0 cm3 [1] recognising 10x dilution / volume of 10 cm 3 pipette 10.0 cm3 pipette 10.0 cm 3 100 cm3 100 cm3 100 cm3 0.100 mol dm-3 0.0100 mol dm-3 0.00100 mol dm-3 pipette 10.0 cm3 1.00 mol dm-3 (10x dilution) (10x dilution) (10x dilution) Procedure 1. Pipette 10.0 cm3 of 1.00 mol dm−3 sodium thiosulfate into a 100 cm3 graduated flask. 2. Top up the graduated flask to the 100 cm3 mark with deionised water, using a dropping pipette to add the deionised water drop-wise when nearing the mark. 3. Stopper the graduated flask and shake the solution thoroughly to ensure that it is homogeneous. The solution is 0.100 mol dm−3 sodium thiosulfate. 4. Repeat steps 1-3 using 10.0 cm3 of 0.100 mol dm−3 sodium thiosulfate to prepare 0.0100 mol dm−3 sodium thiosulfate. 5. Repeat steps 1-3 using 10.0 cm3 of 0.0100 mol dm−3 sodium thiosulfate to prepare 0.00100 mol dm−3 sodium thiosulfate. [1] correct use of pipette + conc of sodium thiosulfate used [1] correct use of volumetric flask

3 Q3(a) General guiding questions • Write an expression to determine the percentage purity by mass of CaCO 3 in the impure sample. What information do you need and how can this be obtained from the experiment? Percentage purity = mass of CaCO3 mass of sample used x 100% The amount and mass of CaCO 3 present in a known mass of impure sample needs to be determined via a back titration experiment. • Why is the back titration method used instead of a direct titration method (refer to Planning Experiments 1 - Volumetric Analysis worked example 1)? What is the use of NaOH in this experiment? Since CaCO 3 is insoluble in water, a back titration is preferred as difficulties, e.g. accurate end- point determination, would arise if it were to be titrated with HC l directly. NaOH is required to react with the excess unreacted HCl in the back titration. m g sample containing CaCO3(s) + FA 1 HCl(aq) x cm3 1.00 mol dm–3 (excess) reaction → some HCl(aq) remained H2O → HCl(aq) 250 cm3 graduated flask pipette → HCl(aq) 25.0 cm3 conical flask titration → FA 2 NaOH(aq) y cm3 0.100 mol dm–3 CaCO3 + 2HCl → CaCl2 + CO2 + H2O HCl + NaOH → NaCl + H2O Key Points Remarks Weighing of impure sample • Method: Direct weighing or weighing by difference (weighing by difference method is used as sample is insoluble in water) • Recording: Mass measurements • Apparatus: Weighing bottle, analytical balance Reaction of calcium carbonate • Ensure complete reaction with HCl  HCl must be in excess • Measurement of HCl  volume needed and precision of apparatus • Reaction between CaCO3 and HCl is complete when effervescence ceased. o Should the solid be added to FA 1 or FA 1 be added to the solid? (solid should be added to FA 1. Addition of FA 1 from the burette to the solid will result in greater loss of chemicals due to acid spray) • Loss of chemicals due to acid spray must be minimised (how?) Preparation of sample solution and use of graduated flask • Is there a need to prepare a solution using a graduated flask? If so, what is the capacity of the graduated flask to be used? (Graduated flask should be used – no need to prepare fresh sample for each titration. Capacity, usually 250 cm 3, should be chosen to ensure sufficient solution for a few titrations to obtain consistent results) • Ensure quantitative transfer (how?) • Apparatus: 250 cm 3 conical flask, burette (for HC l), filter funnel, glass rod, 250 cm 3 graduated flask. Titration • Method: Back titration • Apparatus: Pipette (usually 25.0 cm3), burette, conical flask • Identity of solutions placed in conical flask or burette • Choice of indicator and end-point colour change • Consistent titre values (within 0.10 cm3) • Recording: Measurement of burette readings/titration results Pre-calculations (You may find the diagram below useful for your pre-calculations) • Determine the volume of reagents to be used o Set the titre value (i.e. 20.00 cm3 FA 2 used) o Set the mass of impure sample used (i.e. 1.50 g) o Volume of FA 1 by considering the amount of HCl that reacted with CaCO3 and the amount of HCl present in excess in the graduated flask. • Are there any assumptions that need to be made? (The impure sample contains 80% by mass of CaCO 3 and any impurities present do not react with HCl and NaOH.) Field Code Changed

4 • Percentage purity = mass of CaCO3 mass of sample used x 100% The amount and mass of CaCO3 present in a known mass of impure sample needs to be determined from the experiment. • Since CaCO3 is insoluble in water, a back titration is preferred as difficulties, e.g. accurate end-point determination, would arise if it were to be titrated with HCl directly. • For the back titration, HCl (in excess) is required - to react with all the CaCO3 present in the sample - to react with NaOH in the titration • To