Number of moles= Mass ×Mr, where Mr is the relative molecular mass of the substance of interest. Note that the mass is calculated in terms of grams (1 kg= 1000 g), while the Mr can be found using the periodic table.
Mass of iron
= 2 kg
= 2(1000)
= 2000 g
Mr of iron= 55.8
Number of moles
= 2000(55.8)
= 111600 mol
Additional:
To learn more about stoichiometry, do check out the following!
