Which has a slightly greater mass in a nuclear reaction, the reactants or the products? why?

The reactants have a slightly greater mass.  In a nuclear reaction, a small amount of mass is converted to energy according to the equation E = mc2.   The difference in mass is referred to as the mass defect.

The Formula of Acetone is 
To find the Mass Molar of a compound, you add the Atomic weights of it's atoms.
If you check the Periodic table:
Atomic Weight of Carbone C: 12.0107
Atomic Weight of Hydrogen: 1.00794
Atomic Weight of Oxygen: 15.9994 

So, you got:
Total Atomic Weight of Carbone in Acetone: 12.0107 * 3 = 36.0321 
Total Atomic Weight of Hydrogen in Acetone: 1.00794 * 6 = 6.04764
Total Atomic Weight of Oxygen in Acetone = 15.9994 * 1 = 15.9994

Now, you add this numbers
36.0321 + 6.04764 + 15.9994 = 58.07914

So, the Molar Mass of Acetone is 58.09714 g/mol

Hope this Helps :D