For thousands of years, the mineral chalcocite has been a highly prized source of copper. Its chemical composition is 79.85% Cu, and 20.15% S. We assume that we have 100 g of the compound which gives 79.85 g Cu & 20.15 g S. Using the atomic mass of Cu (63.55 amu) & 79.85 g Cu, the moles of Cu is (1.256, 2.490, 0.796) while atomic mass of S (32.07 amu) & 20.15 g S gives the mole of S as (0.6283, 0.3171, 1.592). Dividing each of the moles by the smallest of the moles, the empirical formula is (CuS, Cu2S3, CuS2, Cu2S)