A Chemistry major obtains a high-paying summer job in an analytical laboratory a

Question

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. He/she then injects 2.0 ?L 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 an area of 1212. The student then makes up several standard ethanol solutions ranging from 0 to 30% ethanol (vol %), injects 2.0 ?L 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.

Explanation / Answer

You basically just have to use these equations A425 = E (yellow) *C + E(blue) * C. A580 = E (yellow) *C + E(blue) * C. Where E is the molar absorptivity, A is the absorbance at a specific wavelength, and C is the concentration. Beer's law is A = E*L*C and here L, the path length, is just 1 cm. These is the same as the equations that we have above. The reason your adding these two absorbances is because both the yellow and blue dyes are in the mouthwash. So now you have to equations and two unknowns and all you have to do is some simple algebra and you should get the concentrations.

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