When solving for the mass of a compound when you’re given the number of moles present, you need to know the molar mass (how many grams there are in a mole of that compound).
In this case, we can make the following equation:
3.00(14.01 + 2(16.00))
In Nitrogen, the molar mass is 14.01 grams per mole, and Oxygen is 16.01 grams per mole.
However, because there are 2 oxygen atoms present per molecule, you must multiply it by two in order to solve for the molar mass.
Anyway:
3.00(14.01 + 2(16.00))
Simplify:
3.00(14.01 + 32.00)
3.00(46.01)
Multiply:
3.00(46.01) = 138.03
3.00 moles of NO2 has a mass of 138.03 grams.
-T.B.
