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Standardization of NaOH and Analysis of Unknown Acid Sample.

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Introduction

Alex Kaplan 20135364 29-SEPT-2003 IB Chemistry II Lab Report: Standardization of NaOH and Analysis of Unknown Acid Sample Introduction: Titration is a process that involves measuring the exact volume of a solution with a known concentration and reacting the solution with a measured volume of a solution with an unknown concentration, or with a weighed sample of a solid. The solution with a known concentration is called the standard solution. To standardize sodium hydroxide (NaOH), one must first test the actual concentration of it, because NaOH is usually of questionable purity. Once the NaOH is standardized using the help of the potassium hydrogen phthalate (KHP) indicator through titration, the NaOH can then be used to determine the identity of an unknown acid sample. The indicator dye will turn pink when the solution suddenly becomes a base (indicating that enough of the titrant has been dripped into the solution to cause this reaction), and the amount of NaOH needed (hence, its concentration) will become evident. The new 'standard solution' will be used to then begin another process of titration, to discover the identity of H(A), the unknown acid. Purpose: The purpose of this experiment is to identify an unknown acid through the titration and standardization of a sodium hydroxide solution. When titrated using the standardized NaOH solution, the identity of the unknown acid will become evident. ...read more.

Middle

13. Calculate the concentration of NaOH given the volume of NaOH used, and the fact that the reaction between NaOH and KHP is of 1:1 stoichiometry. Average the values together to find the concentration of the NaOH mixture. Data and Observations: Value (Uncertainty) Flask 1 Flask 2 Flask 3 Mass of KHP .637g (?0.001) .600g (?0.001) .598g (?0.001) Volume of Water 100mL (?0.1) 100mL (?0.1) 100mL (?0.1) Starting Volume of NaOH 50mL (?0.02) 50mL (?0.02) 50mL (?0.02) Ending Volume of NaOH 16.46mL (?0.02) 18.77mL (?0.02) 18.85mL (?0.02) Observed variances: The cork broke in our 1-L bottle of NaOH, possibly tainting our results (after sample 2 was titrated). Sample 2 was a darker pink relative to flask 1 and 3. Flask 1 and 3 were relatively the same shade. Calculations: Trial I: .637g KHP x (1 mol KHP / 204.23g per mol) = .003119 mol KHP .003119 mol KHP x (1/ .03354 L NaOH) = .09283 M NaOH Trial II: .600g KHP x (1 mol KHP / 204.23g per mol) = .002937 mol KHP .002937 mol KHP x (1/ .03123 L NaOH) = .09398 M NaOH Trial III: .598g KHP x (1 mol KHP / 204.23g per mol) = .002928 mol KHP .002928 mol KHP x (1/ .03115 L NaOH) = .09399 M NaOH Average Concentration: 0.09370 M Part II: Identification of H(a), The Unknown Acid Materials: * Buret and clamp * 5-mL pipet and safety bulb * 1-L glass or plastic bottle with stopper (filled with NaOH solution from part I) ...read more.

Conclusion

x (1/.00255 mol H(a) ) = 207.54g/mol H(a) Trial II: .03138 L NaOH x 0.0937 M NaOH = .00294 mol NaOH or H(a) 0.617g H(a) x (1/.00294 mol H(a) ) = 209.86g/mol H(a) Trial III: .03478 L NaOH x 0.0937 M NaOH = .00326 mol NaOH or H(a) 0.670g H(a) x (1/.00326 mol H(a) ) = 205.59g/mol H(a) Average molar mass: 207.66g/mol H(a) Conclusion: While there were many mistakes made during the execution of this lab experiment, overall, the results did not deviate extremely far from the norm, or expected result. We determined that the unknown acid, H(a), was actually the acid used during the first set of titrations in Part I, potassium hydrogen phthalate. We were off by 3.46 g/mol, probably due to poor titration skill. The group let the solution become too pink, or too saturated. Another possible reason that the error can be attributed to was the fact that we filled the buret up to the 50mL mark, as opposed to filling it up partway and taking an accurate reading. While we let out enough of the NaOH solution to make it appear as if it were exactly at the 50mL mark, this was not the appropriate way to set up the titration lab. The colors were not uniform, and the average did not provide an accurate representation of the real-world concentration of NaOH nor the actual concentration of KHP in both instances, when it was known and unknown. Finally, the fact that cork dropped into our NaOH solution could possibly the culprit of the inaccuracies. ...read more.

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