Relative Atomic Mass and Relative Molecular Mass (5)

Concept of Relative Formula Mass, RFM (1)

The concept of relative mass is also used for ionic substances

Concept of Relative Formula Mass, RFM (2)

The relative mass of an ionic substance is called the relative formula mass, RFM

Concept of Relative Formula Mass, RFM (3)

The relative formula mass is calculated by summing up the relative atomic masses of all the atoms shown in the formula of the ionic substance

Concept of Relative Formula Mass, RFM (4)

This is because the mass of an ion does not differ much from the mass of its atom that forms the ion

The calculation of relative formula mass, RFM of zinc chloride, ZnCl2

RFM of zinc chloride, ZnCl2 = RAM of Zn + 2(RAM of Cl) = 65 + 2(35.5) = 65 + 71 = 136

The calculation of relative formula mass, RFM of aluminium sulphate, Al2(SO4)3

RFM of aluminium sulphate, Al2(SO4)3 = 2(RAM of Al) + 3[RAM of S + 4(RAM of O)] = 2(27) + 3[32 + 4(16)] = 54 + 3[96] = 342

The calculation of relative formula mass, RFM of H2

RFM of water, H2 = 2(RAM of H) = 2(1) = 2

The calculation of relative formula mass, RFM of O3

RFM of oxygen gas, O3 = 3(RAM of O) = 3(16) = 48

The calculation of relative formula mass, RFM of CO

RFM of CO = RAM of C + RAM of O = 12 + 16 = 28

The calculation of relative formula mass, RFM of NH3

RFM of NH3 = RAM of N + 3(RAM of H) = 14 + 3 (1) = 17