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To calculate the mass of one mole of a compound, the number of each type of atom in the compound is multiplied by that atoms relative atomic mass and all those numbers added together. This value is called the relative formula mass (or relative molecular mass or molar mass) of a compound

Note r.f.m, r.m.m, or Mr have no units because they are ratios. The molar mass is obtained from r.f.m, r.m.m, or Mr by simply adding g (grams)

If all the individual atomic masses of all the atoms in a formula are added together you have calculated the relative formula mass (for ionic compounds) or molecular mass (for covalent elements or compounds), Mr. can be used for any element or compound

Whereas relative atomic mass (above) only applies to a single atom, anything with at least two atoms requires the term relative formula/molecular mass. Examples, calculate the formula/molecular mass of  diatomic molecules of the compound contain hydrogen H2 and chlorine Cl2.


Relative atomic masses, Ar: H = 1, Cl = 35.5

R.M.M 2HCl = H (2×1) + Cl (2×35.5)

R.M.M of HCl  = 73.


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