A 4.97 g sample of petroleum was decomposed by wet-ashing and then diluted to 500 mL in a volumetric flask. Cobalt was determined by treating 25.00 mL aliquots of this diluted solution with an organic molecule, called a ligand, that binds cobalt with a colour change (colourless to green).

The data is shown in the table below:

Reagent volume (mL)

Co2+ (3.00 ppm) Ligand (0.010 M) H2O Absorbance (600 nm)

0.00 20.00 5.00 0.398

5.00 20.00 0.00 0.510

It is known that the Co2+-ligand complex obeys the Beer-Lambert Law.

Calculate the mass percentage of the cobalt in the original sample.