Hi kitty I'm more than happy to help
In this question you're given the equation because you will need it when applying the concept of stiochiometry:
n (O2) / 5 = n (NH3) / 4
If you realize 5 is the number next to the oxygen gas in the equation: this indicates that for every 5 Oxygen gas molecules you will need 4 ammonia molecules. Sometimes, they give you the equation not balanced so you have to balance it!
From here we can calculate the number of moles for the oxygen gas
By rearranging this equation: n (O2)/ 5 = n (NH3) / 4 you get the following: n (O2) = 5 n (NH3) / 4
Now all what you have to do is substitute the number of moles of NH3 in the equation and you will get the following:
n (O2) = 5 x 3.315 moles / 4 = 4.14375 moles
But the question didn't end here because they're asking about the mass and not the number of moles. So, we will use the number of moles to convert in to mass by using the molar mass of O2
Molar mass of O2 = 2 x Molar mass of Oxygen = 2 x 16g/mol = 32 g/mol
mass (O2) = n (O2) x M (O2) = 4.14375 mol x 32 g/mol = 132.6 g of O2
Answer: 132.6 g of O2
I hope the explanation helped you, Have a good day and good luck!