Step 1
The reaction must be written and balanced:
Cu2S (s) + O2 (g) => Cu (s) + SO2 (g)
Remember: the number of atoms of all species must be the same on both sides of the reaction.
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Step 2
There is 1 S on both sides
There are 2 Cu on the right, so we multiply on the right by 2
Cu2S (s) + O2 (g) => 2 Cu (s) + SO2 (g)
There are 2 atoms of O on both sides, therefore we only add a number 1.
1 Cu2S (s) + 1 O2 (g) => 2 Cu (s) + 1 SO2 (g)
Answer: The molar coefficient for O2 = 1
