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Unformatted text preview: CHEM 209
10/3/05 NAME: 1. (30 POINTS) (a) A Chemistry major obtains a high-paying summer job in an _
analytical laboratory and is given the task of determining the ethanol content (in vol %)
of a commercial mouthwash sample. He/she decides to do this by gas chromatography
because the experiment was so enjoyable in Chem 209. Accordingly, the student takes
exactly 2.0 mL of the mouthwash sample and dilutes this to 25.0 mL with deionized
water. Hefshe then injects 2.0 pL of the diluted sample into the GC and obtains a
chromatogram containing two peaks. The ﬁrst peak, which corresponds to ethanol, has a
retention time of 75 seconds and an area of 216 while the second peak, which
corresponds to water, has a retention time of 117 seconds and anarea of 1212. The student then makes up several standard ethanol solutions ranging from 0 to 30%
ethanol (vol %), injects 2.0 pL of each into the GC, and uses the areas of the ethanol peak to create a calibration curve whose linear regression equation is: y= 144.2 x Calculate the ethanol concentration (vol %) of the mouthwash sample. SHOW ALL _
WORK. (b) Suppose that chromatograrn obtained for an unknown'sample contains six distinct
peaks. _ What is the minimum number of different species in the sample?
What is the maximum number of different species in the sample? Brieﬂy indicate two reasons why these two numbers might not be the same. __...__.___..—.~——__....
mm ——___._..._..—.-———.—.._...t.... 2. (10 POINTS) Write the equilibrium constant expressions for the following chemical
reactions: -(a) H80; + 13‘ + 3H20 -—--> HSO4‘ +3I' + 2H30+ K: (b) Ag + cr --——+ AgCle (solid) K: 3. (30 POINTS) (3) Write the balanced chemical reaction that occurs during the complete
titration of Na2C03 with HCl. (b) On the graph below, sketch the shape of the titration curve expected for this
titration. mL H01 (c) If the NaZCOs weight taken for the titration was 0.3333 gram and concentration of the
HCl solution was 0.096M, calculate the equivalence point volume expected. SHOW ALL WORK CLEARLY. (Formula weights: Na2C03 = 105.99 g/mole; HC1= 36.46 g/mole) 4. (20 POINTS) Draw block diagrams of each of the following analytical instruments,
labeling all the important components. (a) a gas chromatograph (GC) (b) a visible spectrophotometer (e.g., the Spectronic 20) 5. (10 POINTS) Calculate the following and SHOW YOUR WORK: (3) the pH of a 0.025M NaOH solution. (b) the %T of a solution whose absorbance is 0.90. "iwmkhmmw—u——u..__..._.._._..‘......_._..‘.....» m - ...
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