Assume that:
n = number of moles (mol)
m = mass of substance (g)
M = molar mass (g/mol)
In #O_2#, the number of moles (n) has been provided for you = 5 moles.
In order to find the mass (m) of #O_2#, you first have to find the molar mass (M) of #O_2#.
Use this formula to help you: #n = m -: M#.
You know that #O_2# is composed of 2 atoms of Oxygen.
If you look in your periodic table, the molar mass (is the atomic weight) of Oxygen is 16.0 g/mol.
Since there are 2 atoms in #O_2#, you have to multiply the number of atoms provided by the molar mass of each atom. Which is 2 #xx# 16.0 g/mol therefore it will give you 32.0 g/mol as the molar mass (M).
Your final step is to find the mass (m) of #O_2#.
Refer back to this formula to help you: #n = m -: M#. Now you have to make #m# as the subject (I'm not sure whether have you learned making an alphabet as a subject in Algebra).
To make #m# as the subject, send #M# to the left hand side therefore it will be multiplied by #n#. Now your formula should look like : #n xx M = m#.
Therefore, 5 moles #xx# 32.0 g/mol = 160 grams is the mass (m) when there are 5 moles of #O_2#.
(I'm so sorry if my explanation is too long, but I'm trying my best so you could understand.)
