Hg(NO3)2 is the chemical formula of mercury(II) nitrate.
Mercury(II) nitrate.
For this you need the atomic (molecular) mass of Hg(NO3)2. Take the number of grams and divide it by the atomic mass. Multiply by one mole for units to cancel. Hg(NO3)2=324.6 grams8.50 grams Hg(NO3)2 / (324.6 grams) = .0262 moles Hg(NO3)2