Can someone explain how it is 1?

Explanation:

You have 112 g of Fe.

The atomic mass of Fe is 56 g/mol

112g Fe/ 56g/mol = 2 mol Fe  < you have to convert to moles of iron(iii) sulfate

From the balanced equation, you get that 2 moles of Fe react to produce 1 mole of Fe2(SO4)3

So, to convert:

2 mol Fe* (1 mol Fe2(SO4)3 / 2 mol Fe) = 1 mol Fe2(SO4)3

moles of Fe cancel and the answer is 1 mole of iron(iii) sulfate.