Lecture 6 Experiment 4 - Determining % Carbon and Purity - PDF

Experiment 4 Analysis of Sodium Hydrogen Carbonate by Gas Evolution Ideal Gas Law PV = nRT  Reaction Equation: NaHCO3(s) + H2SO4(aq) ➞ H2CO3(aq) + NaHSO4(aq) HCO3— ion is amphoteric When the reaction occurs, CO2 is evolved. Where does CO2 come from? H2CO3(aq) decomposes The equation for the r...

Experiment 4 Analysis of Sodium Hydrogen Carbonate by Gas Evolution Ideal Gas Law PV = nRT  Reaction Equation: NaHCO3(s) + H2SO4(aq) ➞ H2CO3(aq) + NaHSO4(aq) HCO3— ion is amphoteric When the reaction occurs, CO2 is evolved. Where does CO2 come from? H2CO3(aq) decomposes The equation for the reaction observed is: NaHCO3(s) + H2SO4(aq) ➞ H2O(l) + CO2(g) + NaHSO4(aq)  Determining % Carbon Use Ideal Gas Law to find moles CO2 (n) PV = nRT Use Dalton’s Law to find Pressure of CO2 (P) Ptotal = PCO + PH O 2 2 P CO = Atm P – Water VP 2 Temperature (T) of CO2 = Room Temp TCO = Water Bath Temp 2 Volume (V) of CO2 V CO2 = Volume of water NaHCO3(s) + H2SO4(aq) ➞ displaced by the gas H2O(l) + CO2(g) + NaHSO4(aq) in the Mohr buret Determining %C NaHCO3(s) + H2SO4(aq) ➞ H2O(l) + CO2(g) + NaHSO4(aq) Calculating %C: ▫ First: Definition of %C ▫ Second: Use your data!! ▫ Recall: Mole ratios between reactants and products are all 1:1 ▫ Realize: All C in the NaHCO3 is converted to CO2  1 mol CO2 = 1 mol C  1 mol C = 12.01 g C ▫ Work backwards:  g C in %C calculation comes from mol C  mol C comes from mol CO2  mol CO2 comes from Ideal Gas Law Determining %Purity NaHCO3(s) + H2SO4(aq) ➞ H2O(l) + CO2(g) + NaHSO4(aq) Remember: %C in a pure sample is always the same ▫ Remember: theoretical Yours isn’t pure! Don’t report a perfect answer! For pure NaHCO3: %Na + %H + %C + %O = 100% actual %C = [g C/g NaHCO3] x 100% for 1 mole of NaHCO3 actual theoretical What should be in your lab notebook? Statement of Purpose – complete sentences at least one of which should include “Ideal Gas Law” Procedure – should refer to the diagram of the apparatus in the lab manual OR draw it yourself! Observations ▫ Appearance of sample initially – dry, white? ▫ What color was the desiccant from last week – blue, pink , purple? What mass of H2O was lost? ▫ Gas evolved – slowly, vigorously? What happened to the water level in the buret? ▫ Did a larger volume of gas evolve from the bigger sample? What should be in your lab notebook? Data recorded ▫ Pressure (+ 0.1 mm Hg) ▫ Volume (+ 0.1 mL) Sample 1 Sample 2 Sample 3 Final volume -Initial volume Vol gas evolved ▫ Temperature (+ 0.1 °C) What should be in your report? Fill in the Report Sheet ▫ Show calculations with formulas and unit labels ▫ Pay attention to Significant Figures Conclusion (3rd person; past tense; passive voice) ▫ Brief overview of procedure with reaction equation ▫ Results explicitly stated  % C (expect 10-13 %C)  % Purity (expected ?) ▫ Explanation of Results  Compare experimental %C with theoretical %C  why isn’t the sample 100% pure?  How precise were your results? (standard deviation) Extra questions for Experiment 4 in files Next Week Experiment 5: - has prelab questions, - has extra questions, - and you need to prepare your own data tables (see page vi)