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