Question

How many molecules and what mass of NO is produced if 138.0g of NO2 is combine with 1.20 e24 molecules of water according to the following reaction: 3NO2+H20--->2HNO3+NO

Answer 1

You have a limiting reagent problem. I know that because amounts for BOTH reactants are given.
Convet 138.0g NO2 to mols. mols = grams/molar mass.
Do the same for 1.20E24 molecules H2O. mol = 1.20E24/6.02E24 = ?

Using the coefficients in the balanced equation, convert mols NO2 to mols NO.
Do the same for mols H2O.
It is likely that the two answers will not be the same which means one of them is wrong. The correct answer in limiting reagent problems is ALWAYS tje smaller one and the reagent producing that value is the limiting reagent.
Now convert mols NO to g. g = mols x molar mass.
For # molecules, remember that 1 mol contains 6.02E23 molecules..