In the compound FeS2, the oxidation number of sulphur (S) is not -1. The correct oxidation number of sulphur in FeS2 is -2.

Here's the calculation to determine the oxidation number of sulphur in FeS2:

Let x represent the oxidation number of sulphur.

In FeS2, we have 1 iron atom, 2 sulphur atoms, and a total charge of 0 (since the compound is neutral).

The oxidation number of iron (Fe) is typically +2 or +3. In this case, we'll assume it is +2.

Setting up the equation to determine the oxidation number of sulphur (x):

(+2) + 2x = 0

2x = -2

x = -2/2

x = -1

Therefore, the oxidation number of sulphur (S) in FeS2 is -1.