So, the empirical formula means the smallest value, like the lowest common denominator in maths, but instead of fractions we are going to be using chemical formulae. I always find it easiest to set it out like a table, with the two elements involved in Copper sulphide at the top. Cu S12.7g 3.2gdivide each value by their molar mass, to get number of moles in each element. (12.7/62, 3.2/32) 0.2 0.1then divide each number of moles by the smallest number of moles calculated, to get the ratio. (0.2/0.1 =2, 0.1/0.1 = 1)2:1Therefore the compound will have two Cu atoms for every S atom, and the empirical formula will be Cu2.