An unknown was prepared with the concentration of 0.000630 M. A Beer's Law plot was prepared using the absorbance values from standard solutions of ASA and a line drawn through all the points passed through the origin with a slope of 1550.2 M–1. The expected absorbance and %T values for the diluted aspirin solution prepared by the student is
and , respectively.
The Beer's law plot is usually absorbance (y axis) against concentration (x axis) so the slope is
absorbance (no units)/concentration (mol litre-1)
so the units of the slope are litre mole-1
Thus to find the absorbance of an unknown we need to multiply the concentration of the unknown (0.000630 mole litre -1) by the slope (1550.2 litre mole-1). The units will cancel to give no units which is in reality 'absorbance'.