Chromatography_Problem_Set_Key

Chromatography_Problem_Set_Key - Chemistry 41 Fall 2004...

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Chemistry 41 Fall 2004 Chromatography Problem Set Key 1. Green Slime, an uncharged organic dye, is present in a 600.0 mL sample of water at an unknown concentration. The sample is extracted with 75.00 mL of cyclohexane. After the extraction, the absorbance of the cyclohexane phase, measured at 635 nm in a 1.00 cm cell. Using Beer’s Law you determine that the concentration of Green Slime in the cyclohexane phase after the extraction step is 1.26 x 10 -5 M. Calculate the concentration of Green Slime in the water sample prior to the extraction step. (Ans: 1.97 x 10 -6 M) Molar absorptivity (ε 635 ) of Green Slime in cyclohexane = 68500 M -1 cm -1 Partition coefficient (K) of Green Slime between cyclohexane and water = 32 0.20 600 75 32 1 1 V V K 1 1 q aq org = + = + = From the above calculation it can be seen that 20 % (0.20) of the green slime is left behind in the aqueous phase. This means that 80 % (0.80) is extracted into the cyclohexane phase. 1-q = fraction extracted 1 – 0.20 = 0.80 This calculation (1 – q) tells us that 80 % of the TOTAL NUMBER OF MOLES OF GREEN SLIME has been extracted into the cyclohexane phase.
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