The density is the ratio between mass and volume. So the mass is the product between density and volume. But! You have to control the units of measure.
7.85 g/l = 0.00785 kg/l
41.25 ml = 0.04125 l
Mass = (0.00785) x (0.04125) = 0,00032381 kg ---> 0,32381 g
(I hope this is right )