Hey There!:
2 C2H2 + 5 O2 = 4 CO2 + 2 H2O
2 moles C2H2 ----------- 5 moles O2
35.0 moles C2H2 ------- moles O2
moles O2 = 35.0 * 5 / 2
moles O2 = 87.5 moles
hope this helps!
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