When iron is heated with oxygen, iron oxide is formed, the mass fraction of oxygen is 0.300. Define the formula.

Let us determine the mass fraction of iron in a given oxide:

w (Fe) = 100 – w (O) = 100 – 30 = 70 (%).

Mass ratio of elements in oxide:

w (Fe): w (O) = 70: 30

Taking into account the atomic masses of the elements, let’s move on to the quantitative ratio:

n (Fe): n (O) = w (Fe) / M (Fe): w (O) / M (O) = 70/56: 30/16 = 1.25: 1.875 = 1: 1.5 = 2: 3

Thus, in this compound, there are 3 oxygen atoms per 2 iron atoms, hence the formula for iron oxide: Fe2O3 (iron (III) oxide).

Answer: Fe2O3.