The first step to find the mass of the 8 moles of NH₃ is to find the molar mass of NH₃. The molar mass of any compound is the mass of one mol of it. To calculate it, we have to look for the atomic masses of the elements that the molecule has.
N: 14.01 amu
H: 1.01 amu
Then the molar mass of NH₃ will be:
molar mass of NH₃ = 14.01 + 3 * 1.01
molar mass of NH₃ = 17.04 g/mol
Now that we know the molar mass of NH₃ (the mass of one mol of it) we can easily find the mass of 8 moles.
mass = 8 moles of NH₃ * 17.04 g/mol = 136.32 g
mass = 136.32 g
Answer: the mass in grams of 8 moles of ammonia is 136.32 g