Postlab 4.docx - Determination of Ascorbic Acid in Vitamin...

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Determination of Ascorbic Acid in Vitamin C Tablets by Titration with Potassium Bromate (KBrO 3 ) Post Lab Quantitative observations The concentration of the Standardized Potassium Bromate (KBrO 3 ) Solution: 0.015 M The concentration of the Standardized Sodium Thiosulfate (Na 2 S 2 O 3 ) Solution: 0.058 M 1. Sample Preparation Trial 1 Trial 2 Trial 3 Mass of a weighing paper (g) 0.2195g 0.2194g 0.2156g Mass of a weighing paper and the unknown (g) 0.6281g 0.6427g 0.6616g Mass of the unknown (g) 0.4086g 0.4233g 0.4460g Concentration of the Standardized Potassium Bromate (KBrO 3 ) Solution: 10.0508 g 4 L x 1 mol 167.00 g = 0.015 mol/L Concentration of the Standardized Sodium Thiosulfate (Na 2 S 2 O 3 ) Solution: 28.9701 g 2 L x 1 mol 248.18 g = 0.058 mol/L Mass of the unknown: Trial 1: 0.6281g – 0.2195g = 0.4086g
Trial 2: 0.6427g – 0.2194g = 0.4233g Trial 3: 0.6616g – 0.2156g = 0.4460g 2. Titration I (Burette #1): Rough Titration Trial 1 Trial 2 Trial 3 The initial volume of bromate solution (mL) 0.0mL 0.0mL 0.0mL The final volume of bromate solution (mL) 48.2mL 50.0mL 49.5mL The volume of bromate solution used (mL) 48.2mL 50.0mL 49.5mL # of moles, bromate ions in the solution used 7.23 x 10 -4 mol 7.50 x 10 -4 mol 7.43 x 10 -4 mol # of moles, Br 2 (Total) 0.00217 mol 0.00225 mol 0.00223 mol The volume of bromate solution used : Trial 1: 48.2mL – 0.0mL = 48.2mL Trial 2: 50.0mL – 0.0mL = 50.0mL Trial 3: 49.5mL – 0.0mL = 49.5mL # of moles, bromate ions in the solution used: V BrO 3 - (mL) x 1 L 1000 mL x M BrO 3 - mol L ¿ ) Trial 1: 48.2mL x 1 L 1000 mL x 0.015 mol L = 7.23 x 10 -4 mol Trial 2: 50.0mL x 1 L 1000 mL x 0.015 mol L = 7.50 x 10 -4 mol Trial 3: 49.5mL x 1 L 1000 mL x 0.015 mol L = 7.43 x 10 -4 mol # of moles, Br 2 (Total): 3 x M BrO 3 - mol L ¿ ) x V BrO 3 - (L)
Trial 1: 3 x 7.23 x 10 -4 mol = 0.00217 mol Trial 2: 3 x 7.50 x 10 -4 mol = 0.00225 mol Trial 3: 3 x 7.43 x 10 -4 mol = 0.00223 mol 3. Titration II (Burette #2): Accurate Titration Trial 1 Trial 2 Trial 3 The initial volume of thiosulfate solution (mL) 0.0mL 0.0mL 0.0mL The final volume of thiosulfate solution (mL) 3.5mL 3.0mL 3.6mL The volume of thiosulfate solution used (mL) 3.5mL 3.0mL 3.6mL # of moles, thiosulfate ion used 2.0 x 10 -4 mol 1.7 x 10 -4 mol 2.1 x 10 -4 mol # of moles, Br 2 (Excess) 1.0 x 10 -4

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