[ortrocad]

From the experiment, the empirical formula of the product is Cu₂S.

Post-Lab Questions:
Predict the effect on a of Cu_aS on the following scenarios:
a. Part of the copper reacts with oxygen to produce Cu₂O.
I can't imagine the chemical equation. But I thought subscript a must decrease because the moles of the reactant must be equal to the product, so if there's another product, moles of Cu will be redistributed.

b. If all sulfur has not been driven off
From my understanding, this asks what will happen if not all the sulfur reacted. My answer was that a would increase. I based my answer from adjusting the value of my data. But some answered it would decrease.

[Borek]

You haven't listed all the information, so it is a bit hard to be sure, but - if I were to guess - your answers look OK.

[AWK]

The reaction of Cu₂S with oxygen proceeds with  formation of Cu₂O and SO₂ or Cu₂O, SO₂ and CuSO₄ depending on temperature (but below 800°C)

Imagine reaction:
Cu₂S + O₂ = Cu_1.8S + Cu₂O (unbalanced)
Copper is oxidized partially and sulfur is not oxidized