Homemade spectrophotometer to exemplify Beer-Lamberts law I have built a 12-band spectrophotometer, and i'm currently keen on proving Beer-Lamberts law through a small practical example. 

The solution that I'm going to test with my spectrophotometer, is orange juice that I thin with water. I naturally know I0 and I1, I.e. the absorption, and the path length that the light has to travel. Meaning that I have 2 unknowns, respectively the concentration and the molar absorptivity. 

I am aware that I can figure out the molar absorptivity by doing multiple assays at different concentrations, plot them into a coordinate system and find the slope between the coordinates, which will be equal to epsilon*pathlength. 

Unfortunately, I don't know how I find the molarity concentration of the orange juice that I intend investigate. I do however have the weight and the amount. Will that be sufficient to calculate the concentration molarity?
The molar absorptivity is a proportionality constant. You do not know what is the compound responsible by the color. So, this constant will not be in L/mol.cm but will be in terms of the concentration units. For example, if you use the mass to express the concentration (g/L) the constant will be express in this unit L/g.cm. With this system, you can estimate concentrations and so on. To known the molar absorptivity you must know the concentration in mol/L and it is not possible with this experiment.