Question

H and O2 combine in ration of 1:8 by mass yo form water. what mass of O2 gas would be required to react completly with 4g oh H gas.

Answer 1

they are in ratio 1:8 i.e for 1g of H gas 8g of oxygen is required
therefore for 4g of H gas 8x4 = 32g of oxygen will be required  

Answer 2

8 gram of oxygen. therefore 32 gram of h gas