2 samples of iron 2 sulphide were prepared by (a) heating a mixture of iron filling and sulphur (b) passing hydrogen sulphide gas into a solution of iron 2 chloride and filtering. Analysis of both samples of iron 2 sulphide shows that 0.8g sulfur combined 1.4g of iron while in B 5.5g of iron 2 sulphide contains 2.0 of sulphur show that these result illustrate to the law of constant of composition