Question

1) A stock solution has a concentration of 100 µg/mL. If 20 µL of stock is added to a beaker. What is the mass of the dye in µg​?

2) If the absorbance of the same dye (after diluted with water to 3 mL) is 0.3 AU, what is A/µg​ of the dye used?

Answer 1

1) A stock solution has a concentration of 100 µg/mL. If 20 µL of stock is added to a beaker. What is the mass of the dye in µg?

1 ml or 1000ul has 100ug of dye then 20ul stock?

100ug of dye               in                     1000ul

            ?                                              20ul

100*20/1000   = 2ug

If you take 20ul of stock it has 2ug of dye.

2) If the absorbance of the same dye (after diluted with water to 3 mL) is 0.3 AU, what is A/µg of the dye used?

According to Beer's law/ Beer-Lambert law absorbance & concentration of solute correlated. The absorbance of a solution directly proportional to the concentration of solute. E.g. If the concentration of solute increased by two times then the absorbance doubled i.e. concentration of solute increased two times so absorbance also increases two times. In above problem, the original dye (X) diluted to three times so the absorbance also decreased by three times (0.3).

Absorbance of stock dye solution is 0.3 (absorbance after dilution)*3 (three times diluted)    =      0.9

And absorbance does not have any units.