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CHEM 132 Final Exam Review

CHEM 132 Final Exam Review - Material Reviewed about CHEM...

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Material Reviewed about CHEM 132 Final Exam, scheduled for Thursday, May 13, 2010 The final exam will be a multiple choice exam covering the last six experiments, #7-12. It will test: your math problem-solving ability (the zinc chloride Empirical formula and Calorimetric acid-base stoichiometry experiments) questions; your ability to use experimental observations to determine how a system at equilibrium will respond to an external applied stress or the pH of a solution; and whether you can apply concepts about oxidation-reduction to explain the relative reactivity of metals and halogens. There will be at least 14 questions (at least 2 per experiment), which would make each worth a whopping 87pts; that should suggest to you that they won’t be simple. You will have ~45 minutes to complete it. Empirical Formula of Zinc Chloride Know how to use experimental data to perform necessary calculations leading to an experimental estimate of the empirical formula: Convert mass of zinc chloride dissolved in a known 100.0 mL volume to the mass present in a smaller volume of that solution: A simple ratio does it: 0.316 g = X g ; just cross-multiply and solve 100.0 mL 21.50 mL For X Determine the mass of zinc in that sample by titrating the number of moles of zinc in the sample, keeping in mind that there is 1 mol Zn in 1 mole zinc chloride EDTA binds to Zn (and Mg and Ca) on a one-to-one molar basis; so if you know the #moles of EDTA used to titrate to the indicator end-pt, you know the #moles of Zn present in the sample titrated: Volume EDTA (in L) x molar concentration [EDTA], mol/L = # moles EDTA #moles EDTA = #moles Zn Convert #moles Zn to mass Zn by multiplying by the molar mass of Zn (65.39 g/mol): #moles Zn X molar mass (g/mol) = # grams Zn Calculate the mass of Cl in the titrated sample: it is the difference between the mass of zinc chloride titrated and the mass of zinc in the sample: Mass zinc chloride ) Mass zinc = Mass chlorine Determine the molar ratio of Zn to Cl in the sample titrated. To do this, you have to convert mass Cl to moles of chlorine by dividing by the molar mass of Cl (35.45 g/mol) Then set up a ratio of: #moles Zn titrated /#moles Cl calculated to be in sample; the ratio should be small, whole# ratio, but experimentally never is The end-pt is determined by a change in color of an indicator. Any indicator has the property of being one unique color in one condition (like, when acidic) and another different color in the opposite condition (ie. basic). The indicator here is EBT. When it binds a metal cation, in this case, Zn , it is 2+ pink. When it no longer binds the cation, it is blue.
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The Zn dissociates from the EBT because it can also bind to EDTA, and it does 2+ so more readily to EDTA. So, as zinc is titrated it is pulled off the indicator. As some of the EBT is no longer bonded to zinc, the color takes on a more blue tint...ie. purple. When all of the EBT is stripped of zinc, the color turns blue.
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CHEM 132 Final Exam Review - Material Reviewed about CHEM...

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