how many grams of acetylene?
It's going to be:
(grams O2 / molar mass O2) / coefficient O2 = (grams C2H2 / molar mass C2H2 ) / coefficient C2H2
(g O2 / 32.0 g/mol O2 ) / 5 mol O2 = ( g C2H2 / 24.04 g/mol C2H2 ) /2 mol C2H2
solve for gO2, after plugging in whatever value you're given for g C2H2(acetylene)