Answer:
80g
Step-by-step explanation:
Given parameters: 
Number of moles of O₂ = 2.5moles 
Unknown: 
Mass of O₂ = ? 
Solution: 
To solve this problem, we use the expression below 
 Mass = number of moles x molar mass 
Molar mass of O₂ = 2(16) = 32g/mol 
 So;
 Mass = 2.5 x 32 = 80g