Convert 682.81 liters to milliliters. Multiply 682.81 by 1000 to obtain 682810 milliliters.
Look up the density, in grams per milliliter, of the substance in question. You can obtain the density from a chemistry textbook, or from reputable chemistry websites.
Multiply the value of milliliters that you obtained in Step 1 by the density, to obtain the mass of the substance, in grams.