3. (a) Describe the internal structure of a spectrophotometer (double beam) outlining how it can be used to determine the concentration of certain substances. [5 Marks] (b) Pure hexane has a negligible ultraviolet absorbance above a wavelength of 200nm. A solution is prepared by dissolving 25.8mg of benzene (C6H6:MWT=78.11) in hexane and diluting to 250ml had an absorption peak at 256nm and an absorbance of 0.266 in a 1.000cm cuvette. Find the molar absorptivity (c) of benzene at this wavelength in Mcm1. [3 Marks] (b) A sample of hexane contaminated with benzene had an absorbance of 0.07 at 256nm in a cuvette with a 5.000cm path length. Find the concentration in mg/L. [2 Marks]