CH_301H_Fall_2007_ExamI_Key

CH_301H_Fall_2007_ExamI_Key - 1 CH 301H Fall 2007 Exam I CH...

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1. CH 301H Fall 2007 Exam I 1-7 CH 301H 54785 Fall 2007 September 24, 2007 Professor Campion Exam I 1. In connection with your job as an analytical chemist you are given a sample and asked to determine whether it is a mixture or a pure substance. You supervisor suspects that it is a mixture of two compounds, one that is soluble in water and one that is not, but she is not sure. Propose a scheme that would allow you to confirm your supervisor’s hypothesis. How can you be sure that the proposed pure substances are, in fact, not mixtures themselves? (5 points) Add the substance to a quantity of water and filter the resulting solution or suspension. If the supervisor is correct, the insoluble component will be left behind in the filter paper and the soluble component will go into solution. Evaporate the solution and dry the material in the filter paper to isolate the two components of the mixture. I’m assuming that the mixture is a mixture of two solid substances. To confirm that the two substances are pure substances, it is necessary to show that they cannot be separated into simpler substances by physical means. Since they are solids you can try to extract them into different solvents to see if they separate, sublime them to see if they separate, find a suitable non-aqueous solvent for the water insoluble compound and try to separate by paper chromatography etc. 2. Sodium chloride that is formed from a reaction of sodium vapor and chlorine gas is found to be 39.34% sodium by mass. A 4.86 g sample of sodium chloride that is purified from seawater contains how many grams of sodium? (5 points) mass Na = (mass of sample)(mass fraction Na (mass %/100)) = (4.86 g)(.3934) = 1.911 g
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1. CH 301H Fall 2007 Exam I 2-7 3. Two elements X and Y combine to form two compounds with the following compositions Compound %Mass X % Mass Y 1 22.56 77.44 2 14.87 85.13 If the empirical formula of Compound 1 is XY 3 then what would be a possible empirical formula for compound 2? Might there be more than one possibility and, if so, why? (5 points) Compound 1 (Mass Y/Mass X) = 77.44/22.56 = 3.43 Compound 2 (Mass Y/Mass X) = 85.13/14.87 = 5.72 Ratio (Compound 2) / (Compound 1) = 5.72/3.43 = 5/3 If Compound 1 is XY 3 Compound 2 could be XY 5 , X 2 Y 10 whose X/Y ratios are all 5.72 4. The element zinc has two naturally occurring isotopes. The natural abundances and isotopic masses are Isotope % Abundance Isotopic Mass 113 Zn 4.29 112.904 115 Zn 95.71 114.904 What is the average relative atomic mass of zinc in natural abundance? (5 points) A = 0.429(112.904) + 0.9571(114.904) = 114.8182
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1. CH 301H Fall 2007 Exam I 3-7 5. What observations led Avogadro to conclude that the elemental gases (hydrogen, nitrogen, oxygen and the halogens) were diatomic molecules? (5 points)
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