Question

Determine the ethanol concentration in an unknown sample given the following GC output. A trend line based on calibration data shows the following functionality: (Area %) 0.975 (Wt% Ethanol) 2.5 2 1.5 1 0.5 3 2.5 2 1.5 1 0 0.5 Time (min) Signal (AU)

Answer 1

From the Graph the area of the Larger peak observed can be calculated by

Area = Height x Width_1/2

Where Height is the peak height and width_1/2 is the width of the half portion of height.

So from the above graph height is 2 and width_1/2 is 0.2

So area is 2 x 0.2 = 0.4

Similarly, there is one small peak whose height is 0.1 width is 0.1

so the area is 0.1 x 0.1 = 0.01

So the total area covered by both peaks is 0.4 + 0.01 = 0.41

percentage of area coverd by larger peak is (0.4/0.41) x 100 = 97.56

Now Area (%) = 0.975 (wt % Ethanol)

putting these value

97.56 = 0.975 x wt % ethanol

Wt % ethanol = 97.56/ 0.975

= 100.06 %

Now to get the concetration Multiply the weight percentage with density of ethanol i.e 0.789 g/ ml and divide by its molecular weight i.e 46.07 g/mol

so (100.06/100) X (0.789g/ 1ml) X (1g/ 46.07 mol) X (1000ml/1ml)

= 17.136 mol/ml is the concetration of ethanol