Molar mass:
O₂ = 16 x 2 = 32.0 g/mol
C₃H₈ = 12 x 3 + 1 x 8 = 44.0 g/mol
Balancing chemical equation :
C₃H₈+ 5 O₂ = 3 CO₂ + 4 H₂O
44.0 g C₃H₈ ------------- 5 x 32.0 g O₂
94.4 g C₃H₈-------------- ( mass of O₂)
mass of O₂ = 94.4 x 5 x 32.0 / 44
mass of O₂ = 15104 / 44
= 343.27 g of O₂
hope this helps!