Matt Marthaler Experiment Four Comparison of Two Chloride Determinations Purpose: In this lab, I will compare the use of two different “wet” chemical techniques for the analysis of an unknown chloride sample. The first analysis is a volumetric chloride determination and the second is a gravimetric determination. I will then compare these methods statistically for the percent chloride determination. Procedure: Volumetric Chloride Determination 1) Place about 4 g of AgNO3 in 100 mL of distilled water and let it dissolve. Place this now into a 200 mL volumetric flask and dilute to the mark followed by a thorough mixing. 2) Into 3 250 mL Erlenmeyer flasks, weigh about 0.1 g of the unknown chloride. Dissolve each sample in 50 mL of water. Add 0.03 g dextrin and 5 drops of dichloroflurescein indicator. Titrate the solution with silver nitrate to a pink endpoint. 3) Calculate the percent chloride in the sample. Gravimetric Chloride determination 1) Prepare 3 Gooch crucibles by cleaning with an acid (0.1 M HNO3) and then base (0.1 M NH3). Follow this with a wash by distilled water and then place in a 120 degree oven for an hour. Fit each crucible with a glass fiber filter. 2) Dissolve each sample into a 400 mL beaker with 150 mL of distilled water, then add one mL of 6 M HNO3. 3) Add silver nitrate to precipitate the solution. Heat this without boiling until the supernatant is clear. 4) Filter samples through Gooch crucibles with mild suction. Use warm (0.1 M HNO3) to wash the precipitate five times. Rinse the precipitate with distilled water after this. 5) Dry the crucibles and precipitate in a 120 degree oven for at least two hours before a final weighing. Reactions: Cl- + AgNO3 AgCl(s) AgCl(s) + NO3Ag+ + Cl- Data: Gravimetric Analysis Mass of Gooch and Lid Before Wash (g) 33.0188 34.1303 35.5765 34.2664 Sample 751 2 3 Average Sample Mass of Unknown sample (g) Mass of Filter Paper (g) 1 2 3 Average 0.2009 0.1999 0.2006 0.2005 0.0288 0.0297 0.0301 0.0295 Mass of Gooch, Lid, and Product After Heating (g) 33.4928 34.5552 36.0559 34.7013 Mass of Gooch and Lid After Wash (g) 33.0172 34.1281 35.5763 34.2405 Mass of Cl(g) % Cl in Unknown 0.1105 0.0983 0.1111 0.1067 55.01 49.17 55.43 53.20 Volumetric Determination Sample 1 2 3 Average Unknown Mass (g) 0.1012 0.1004 0.1004 0.1007 Dextrin Mass (g) 0.0304 0.0300 0.0310 0.0305 mL of AgNO3 used to Titrate 14.40 13.99 13.11 13.83 Mass of Cl- (g) 0.06021 0.05850 0.05482 0.05784 % Cl- in Unknown 59.50% 58.27% 54.60% 57.46% AgNO3 mass and mols: 4.0091 g and 0.118 mols Comparison of the Two Methods Sample 1 2 3 Average Standard Deviation: 4.97 Tcalculated: 1.48 Vol. Determination % Cl 59.50 58.27 54.60 57.46 Grav. Determination% Cl 55.01 49.17 55.43 53.20 Difference 4.49 9.10 -0.83 4.26 Calculations: Volumetric Analysis Moles of AgNO3 used: 4.00091 g AgNO3 x 1000 mL x 1 mol AgNO3 = 200 mL x 1 L x 169.87 g AgNO3 Mass of Cl- (Final): 14.40 mL AgNO3 x 4.0091 g AgNO3 x 1 mol AgNO3 x 1 mol Cl x 35.435 g Cl = 200 ml AgNO3 x 169.8682 g AgNO3 x 1 mol AgNO3 x 1 mol Cl % Cl- in Unknown: 0.0602 𝑔 𝐶𝑙 0.1012 𝑔 𝑈𝑛𝑘𝑛𝑜𝑤𝑛 0.118 mol AgNO3 0.0602 g Cl- x 100% = 59.50% Gravimetric Analysis Volume of AgNO3 needed to precipitate the unknown chloride: g unk sample x mol AgNO3 x L AgNO3 x avg %Cl from volumetric analysis mol Cl2 x mol AgNO3 x g Cl Ex: 0.2009 g unk sample x 1mol AgNO3 x 1 L AgNO3 x .5746 %Cl from volumetric analysis 1 mol Cl2 x 0.118 mol AgNO3 x g Cl = 27.61 mL AgNO3 Mass of Product (AgCl): (mass of crubile, lid, and product) – (mass of crucible and lid + filter paper) Ex: (33.4928 g) – (33.0172 + 0.0288) = 0.4468g AgCl Mass of Cl-final: 0.4468 g AgCl x 1 mol AgCl+1 mol Cl x 35.45 g Cl = 143.32 g AgCl x 1 mol AgCl x 1 mol Cl % Cl- in Unknown: 0.1105 g Cl− x 0.2009 g AgCl 0.1105 g Cl- 100% = 55.01% Comparison of the Two Methods Standard Deviation of differences: ∑i (di−mean ds)2 n−1 s=√ ∑(4.49−4.26)2 +(9.10−4.26)2 +(−.83−4.26)2 3−1 Example: s= √ Ttest: tcalc= |𝑑𝑠 | ×√𝑛 Ex: tcalc = 4.26 𝑥 √3 = 1.48 4.97 ̅ = 4.97 Conclusion: The reason this lab was performed was to determine whether the results of %Cl using volumetric and gravimetric determination were comparable. After calculation, we have found that the %Cl was 57.46% from the volumetric determination and 53.20% from the gravimetric determination. In comparing these two results using a t test, I have calculated t to be 1.48. When this is compared to the t table value at the 95% confidence level (3.182), the two methods were found to be not significantly different. Some procedural changes that would improve the error are to have better cleaning procedures of the Gooch crucibles to get rid of all the impurities and to make sure the oven door was closed the whole time to dry the product fully.