# What is the molar Mass of Fe3+, how many ions does it have and what is the mass of ion in one mole of compound?

Mar 11, 2015

The molar mass of Fe³⁺ is 55.845 g/mol.

The only difference between an Fe atom and an Fe³⁺ ion is three electrons.

The electrons have so little mass compared to the rest of the atom that their masses can be ignored.

So, the molar masses of Fe and of Fe³⁺ are the same: 55.845 g/mol.

The mass of Fe³⁺ in 1 mol of compound depends on the formula of the compound.

For example, 1 mol of Fe(NO₃)₃ contains 1 mol of Fe³⁺ ions (55.845 g).

But 1 mol of Fe₂(SO₄)₃ contains 2 mol of Fe³⁺ions (111.690 g).

Mar 11, 2015

The molar mass of the ${\text{Fe}}^{3 +}$ cation is, for all intended purposes, identical to that of the neutral iron atom, which is $\text{55.845 g/mol}$.

The mass of a single electron is approximately 2000 times smaller than those of a proton and a neutron, so you can safely assume that the addition of electrons, as in the case of anions, or the removal of electrons, as in the case of cations, has little to no impact on molar mass.

Moreover, 1 mole of ${\text{Fe}}^{3 +}$ cations contains exactly $6.022 \cdot {10}^{23}$ ${\text{Fe}}^{3 +}$ cations - this is known as Avogadro's number - and weighs exactly 55.845 g.

If you have 2 moles of ${\text{Fe}}^{3 +}$ ions, their weight is going to be twice the weight of one mole; if you have 0.5 moles, the weight of the ions will be half the weight of one mole.

So, unless specifically told otherwise, always assume that the molar mass of an ion is identical to that of the neutral atom.