Question

how many atoms are present in Fe2O3

Answer 1

There are two iron atoms and threeoxygen atoms in each molecule of Fe2O3. So that's five atoms per molecule. One mole of any molecular substance contains 6.02×1023 molecules

Answer 2

1 mole of any substance contains 6.02⋅1023 particles (this is Avogadro's number). That means that 0.550 mol of Fe2O3 contains 0.550⋅6.02⋅1023 formula units of Fe2O3

Each formula unit of Fe2O3 contains 2 atoms of iron and 3 atoms of oxygen (technically they are ions, but that doesn't matter).

So the total number of iron atoms is
(0.550⋅6.02⋅1023) x 2 = 6.622⋅1023


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