The mass percentage of iron in (potassium ferricyanide) is approximately .
Explanation:
Start by finding the formula mass of potassium ferricyanide, , using relative atomic mass data from the question:
.
In other words, every one mole of formula units have a mass of approximately .
On the other hand, note that there is one iron atom in every one mole of formula units. Hence, there would be exactly one mole of iron atoms in every one mole of formula units. The mass of that one mole of iron atoms is approximately (again, from the relative atomic mass data of the question.)
Therefore:
.