Question

Q1.A compound of copper and sulphur is 65.43percent copper by mass.how many grams of copper can be obtained from 249g of this compound?and how many grams of sulphur remain?

Answer 1

Here is a worked example of a stoichiometry problem.
http://www.jiskha.com/science/chemistry/stoichiometry.html
But you can do it a lot faster another way.
249g copper sulfide x 0.6543 = g Cu that can be obtained.
S = 34.57 S and 249 x 0.3457 = g S in the 249 g.