How many moles of O2 are needed for the complete combustion of 3 moles of CH4?

Answer 1

Write and balance the equation, then use the coefficients.
CH4 + 2O2 ==> CO2 + 2H2O

The equation tells you that 1 mol CH4 requires 2 mols O2; therefore, 3 mols CH4 will require......mols O2.