The balanced chemical reaction is written as:
2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(g)
We are given the amount of acetylene to be used in the reaction. This will be the starting point for the calculations.
20.0g C2H2 ( 1 mol / 26.04 g) ( 5 mol O2 / 2 mol C2H2 ) ( 32 g / 1 mol ) = 61.44 g O2 is needed