To find the mass of ammonia (NH3), we first need to calculate the molar mass of NH3.

The molar mass of NH3 is:
N: 14.007 g/mol
H: 1.008 g/mol (3 hydrogen atoms)

Molar mass of NH3 = 14.007 g/mol + 1.008 g/mol * 3 = 17.031 g/mol

Now, we can calculate the mass of 2.5 moles of NH3:
Mass = moles molar mass
Mass = 2.5 mol 17.031 g/mol
Mass = 42.5775 g

Therefore, the mass of 2.5 moles of NH3 is 42.5775 grams.