The mass fractions of iron and sulfur are equal to 46.67% and 53.33%, respectively. Determine the compound formula.

Let us denote: a is the number of iron atoms, b is the number of sulfur atoms, M is the molecular weight of the compound.
The relative atomic masses are known: iron is 56, sulfur is 32.
Let’s compose the following expressions:
0.4667 = 56 * a / M;

0.5333 = 32 * b / M;

Let’s write expressions for molecular weight:
M = 56a / 0.4667 = 120a;

M = 32b / 0.5333 = 60b;

Equating expressions:

120a = 60b. This means that a = 1, b = 2. The formula of the compound is FeS2.

Answer: the substance has the formula – FeS2.