Ar: Relative atomic masses
1) the Ar can be found on the periodic table as the mass number which is the bigger number out of the two numbers shown near an element.
Mr: relative formula mass
1) compounds have relative formula masses.
2) to find the Mr (you must add up all of the Ars' in the compounds elements).
Practical: finding the mr for MgCl2
The Ar for Mg is 24
The Ar for Cl is 35.5 but because there are two atoms we must times that to make 71
Now the Mr is ( 71 + 24 ) = 95
1) The relative formula mass (Mr) or relative atomic mass (Ar) of a substance in grams is known as one mole of that substance.
Put into practise:
1) iron Fe has an Ar of 56 - So one mole of iron weighs exactly 56g
2) Nitrogen gas, N2, has an Mr of 28 (2 X 14) - So one mole of Nitrogen weighs exactly 28g