Relative Formula Mass
The Relative Atomic Mass (Ar) is just like the Mass Number of an element. For example: Heliums Mass Number is 4, so its Relative Atomic Mass is 4 as well.
The Relative Formula Mass (Mr) is just the combined total of all Ar's in a compound. For example:
- MgCl2. Ar of Mg (Magnesium) = 24. Ar of Cl (Chloride) = 35.5. 24 + (35.5 x 2) = 95. 95 is the Mr of MgCl2
One Mole of a substance is equal to its Mr or Ar in grams. For example:
- One mole of Carbon is 12g, as the Ar of Carbon is 12.
- One mole of Nitrogen is 28g, as it travels in pairs (N2), and its Ar is 14.