The relative formula mass is the sum of the relative atomic masses of the atoms in one formula unit of an ionic compound; it is the ionic equivalent of relative molecular mass and has no unit.
The relative formula mass is the sum of the relative atomic masses of all the atoms in one formula unit of an ionic compound. It is the ionic-compound version of relative molecular mass, and like the others it is a ratio with no unit. Malay: jisim formula relatif · Chinese: 相对式量.
Ionic compounds are not made of separate molecules; they are giant lattices of ions, so we describe the smallest repeating ratio of ions using a formula unit rather than a molecule. The relative formula mass adds the Ar values in that formula unit, in exactly the same way you would add them for a molecule, remembering to count each ion as many times as the formula shows.
Example. Sodium chloride, NaCl, has a relative formula mass of 23 + 35.5 = 58.5. Calcium chloride, CaCl2, gives 40 + 2(35.5) = 111. For a hydrated salt such as CuSO4·5H2O, you must also include the five water molecules of crystallisation in the total.
Confusion to avoid. The only real difference between relative formula mass and relative molecular mass is the type of substance. Use relative molecular mass for covalent, molecular substances and relative formula mass for ionic compounds; the arithmetic is the same, but naming an ionic compound’s total a “molecular” mass loses accuracy marks. Do not forget the water of crystallisation in a hydrated salt, since leaving it out is one of the most common slips.
In this chapter the relative formula mass becomes the molar mass in grams per mole for an ionic compound, so it drives every mole calculation involving salts. Because so much of Chemistry deals with ionic substances, this is a term you will use constantly across SPM Chemistry.
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