Question

For 2H2 + O2 → 2H2O: 4 moles of H2 will react with moles of O2 to produce moles of H2O

Answer 1

The ratio is 2:1. You need two moles of hydrogen for every mol of O2. The proportion would be written as
2:1::4:x
2/1 = 4/x Cross multiply
2x = 4 Divide by 2
x = 4/2
x = 2

You would need 2 moles of O2 to react with 4 mols of H2

Answer 2

4 moles of h2 will react with 2 moles of o2 to produce 2 moles of h20