The charge of 96 amu of S2- is -6. The mass of a sulfur atom is 32 amu per atom, and the mass of a sulfide ion is also 32 amu as the weight of gained electrons is negligible. Therefore, the number of sulfide ions in 96 amu is 96/32 = 3. Each sulfide ion has a -2 relative charge, so the total relative charge is -2 x 3 = -6. The exact charge is 6 x e = 9.6 x 10^(-19) C, where e is the elementary charge.