Verified answer

There is a figure in the textbook "Advanced Inorganic Chemistry", by Cotton and Wilkinson, 5th edition (and perhaps the 6th, but I don't have access to that) that shows the electronic absorption spectrum of the Co(H₂O)₆²⁺ ion (which is the species you're observing if the λmax is 510 nm). A very recent measurement appears in a journal article (Inorg. Chem., 2012, 51, 4850-54 – Figure 2). The molar absorptivity at 510 is approximately 5 M⁻¹ cm⁻¹. The absorption is weak because Co(H₂O)₆²⁺ has a center of symmetry and d-d transitions are dipole forbidden for centrosymmetric complexes. In the figure I cited, you'll see that the unusual tetrahedral complex Co(H₂O)₄²⁺ (that's why it was published) has a molar absorptivity about 100 times stronger (marked [emim]BF₄ in the figure) because it is tetrahedral and has no center of symmetry.

If you look up the article (you'll need to be at a location where you can get free access - on campus at a college or university), you should be able to confirm that the spectrum to which I'm referring looks like yours.

EDIT: I put a screen shot of the figure I cited in the link below. The pink line is the Co(H₂O)₆²⁺ spectrum (Note: × 100 !) and the black is the the unusual tetrahedral complex Co(H₂O)₄²⁺.

Hello fellow IB Chemistry Student.

The molar absorptivity is the slope of the line, if you graph Absorption vs. Molarity. I got 4.97M-1cm-1.