Relative atomic mass, Relative molecular mass, and The mole
Relative atomic mass (Ar)
Relative atomic mass = mass of one atom of an element
1/12 the mass of one atom of carbon-12
Approximate relative atomic masses can be found from the mass numbers in Periodic Table
Relative molecular mass (Mr)
The relative molecular mass of a compound is the sum of the relative atomic masses of all the atoms in a molecule of the compound.
e.g: What is the relative molecular mass of sulphuric acid H2SO4?
Mr(H2SO4) = (2 x1) + 32 + (4 x 16) = 98
The mole is a convenient way of describing a large number of objects (in this case atoms, molecules or ions). The mole (abbreviated mol) is the SI unit for amount of substance.
The number of atoms in 1g of hydrogen atoms is 602 000 000 000 000 000 000 000 (6 x 1023 ) and this amount is called one mole. A mole of any substance always contains this number (called the Avogadro number) of particles.
Mole (n) = Mass of substance (m)
Mass of 1 mole substance (M)
e.g: What is the mass of 0.1 mol of water?
Mr(H2O) = 18
1 mol water weighs 18g
0.1 mol water weighs 0.1 x 18g = 1.8g
Calculating reacting masses
What mass of iodine will react completely with 10.0 g of aluminium?
Mass of Al= 10 g
Know the equation reaction of iodine and alumunium
2Al (s) + 3I2 (s) 2AlI3 (s)
This equation tells us that 2 mol of Al reacts with 3 mol of I2; so we can write the ratio: amount of Al = 2/3
amount of I2
Calculate the moles of the reaction
a. mol of Al:
n = m/M
n = 10.0 g/27.0 mol.g-1
= 0.370 mol
b. mol of I2:
n I2 = 3/2 X mol Al
= 0.555 mol
Mass of I2 = n X M
= 0.555 mol X 254 g mol-1
= 141 g