A 1ml solution of a protein in a cuvette gives an optical density of 0.87 at 280nm. The Molar extinction?

Answer 1

OD=E*d*C

Where OD=optical density
E the molar extinction coefficient
d the length of the path of the light through the solution (width of the cuvette)
C the concentration in mole/L of the solute.

You need d and C as well to get E.
This is a fair approximation especially for student problems.

In reality if you wanted to find E, you would do a standard curve by measuring the OD of samples of known concentration.
Then you would use linear regression to find the equation of the curve and from its slope you would calculate E (slope=E*d)

A good standard curve and thus accurate calculation requires care to ensure the accuracy of the standards, several points (ideally 9 at least) and being within the linear are of the method.
The region of highest accuracy is 0.2-0.8. You can go even below or above this range (though not above 1.0) but this is the range where the photometric error is minimal.

Answer 2

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