Mass % question?

Question

Can someone please tell me the mass % of Copper(I)Sulfide and Copper(II)Sulfide? Please explain how you got your answer.

Answer 1

% by mass is simply (Mass from each element) / (Molar Mass)

For this, you will need the chemical formula for each compound.

Copper(I)sulfide = Cu2S

Mass from Copper = 63.5 x 2 = 127 g/mol
Mass from Sulfur = 32 g/mol
Molar Mass = 159 g/mol

% Cu = 127/159 = 80%

% S = 32/159 = 20%

Copper(II)sulfide = CuS

Mass from Copper = 63.5 g/mol
Mass from Sulfur = 32 g/mol
Molar Mass = 95.5 g/mol

% Cu = 63.5/95.5 = 66.5%
% S = 32/95.5 = 33.5%

Answer 2

The average isotopic mass of Cu is 63.546 g/mol
The average isotopic mass of S is 32.065 g/mol

Copper(I)sulfide is Cu2S (Cu+ and S 2-)
Copper(II)sulfide is CuS (Cu 2+ and S 2-)

Do you mean mass % of copper in each? If so, then for Cu2S it is:

63.546/(63.546 + (2 x 32.065) = 0.4977 = 49.77 % copper

For CuS it is

63.546/(63.546 + 32.065) = 0.6855 = 68.55 % copper.

If you are after the sulfur mass % instead, then simply subtract the above answers from 100 %.

Cheers