Quiz 2 Key - FW (PNCl 2 ) = (31.0) + (14.0) + 2(35.5) =...

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CHEMISTRY 201X: Quiz #2 August 31, 2007 Name: _______KEY_________________________ (1) (2 pts) What is one important difference between SEM (scanning electron microscopy) and TEM (transmission electron microscopy) in how the images are obtained? SEM: reflected electrons (back-scattered, secondary, Auger) TEM: transmitted electrons (elastically and inelastically scattered) (2) (2 pts) On the backscattered SEM image of steel below, some regions are richer in carbon ( Z = 6) while others are richer in chromium ( Z = 24). Using arrows, identify 2 regions richer in chromium. Light regions = higher Z ; Dark regions = lower Z . Chromium-rich regions should appear lighter… (3) (3 pts) A new candidate for wire insulation has an empirical formula of PNCl 2 . What is the percent (by mass) of phosphorus in this compound?
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Unformatted text preview: FW (PNCl 2 ) = (31.0) + (14.0) + 2(35.5) = 116.0 g/mol (=116.0 amu) % Mass P in PNCl 2 = 2 31.0 g P 116.0 g PNCl 100% = 26.7 % Ans. ___ 26.7 % P ______ (4) (2 pts) Using the mass spectrum below, what is the molecular formula of this compound? (Use the following isotope abundances: 31 P, 100%; 14 N, 100%; 35 Cl, 75%; 37 Cl, 25%) mass/charge (amu) 345 347 349 351 353 355 357 359 Relative Abundance 0.0 0.1 0.2 0.3 0.4 Peaks occur near 348 amu = 3 116 amu. Molecular formula contains 3 empirical formulas: P 3 N 3 Cl 6 Ans. ____ P 3 N 3 Cl 6 _____ (5) (1 pt) Explain the pattern of peaks in the mass spectrum. 2 isotopes of Cl distributed over 6 Cl positions should give 7 peaks ; only 5 observed. Lighter molecular isotopes are more abundant than the heavier isotopes....
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