how many moles c2h2 will be needed to fully react with 7.40 moles of o2

Answer 1

2C₂H₂ + 5O₂ ==> 4CO₂ + 2H₂O ... balanced equation for combustion of C₂H₂

Once you have the correctly balanced equation, you can use dimensional analysis to answer the question:

7.40 moles O₂ x 2 moles C₂H₂ / 5 moles O₂ = 2.96 moles C₂H₂ needed