1. Find the molar mass of NO2:

* Nitrogen (N) has a molar mass of 14.01 g/mol

* Oxygen (O) has a molar mass of 16.00 g/mol

* NO2 has one nitrogen and two oxygen atoms: (14.01 g/mol) + 2*(16.00 g/mol) = 46.01 g/mol

2. Use the molar mass to convert moles to grams:

* Mass = moles * molar mass

* Mass = 3.00 mol * 46.01 g/mol

* Mass = 138.03 g

Therefore, 3.00 moles of NO2 have a mass of 138.03 grams.