How do I find a true mass corrected for buoyancy?

I weighed several samples of a chemical in chemistry lab. I am told to correct the masses that the scale gives in order to find the true mass, since the mass reading on the scale is affected by the buoyancy of the air.
Does anyone have an equation that can compute the corrected mass? Something along the lines of:

true mass = measured mass * (?)

Additional Details

The substance is the base TRIS.
I measured 0.42 grams.
The density of TRIS is 1.327 g/mL.
The molecular mass of TRIS is 121.14 g/mol.
The density of air is 1.2 kg/meter^3. assuming you know what the substance is made of (carbon, oxygen, phosphate?), you can use the periodic table to calculate the density because you know the mass.

or vice versa?