WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.3 Chemical calculations

Relative mass terms Masses of atoms

The masses of individual atoms are too small to be used in calculations in chemical reactions, so instead the mass of an atom is expressed relative to a chosen standard atomic mass. The carbon-12 isotope is taken as the standard of reference because relative atomic masses are determined by mass spectrometry and volatile carbon compounds are widely used in mass spectrometry. Most elements exist naturally as two or more different isotopes. The mass of an element therefore depends on the relative abundance of all the isotopes present in the sample. In order to overcome this, chemists use an average mass of all the atoms and this is called the relative atomic mass , A r . Relative atomic mass has no units since it is one mass compared with another mass. If we refer to the mass of a particular isotope then the term relative isotopic mass is used. Masses of compounds Since the formula of a compound shows the ratio in which the atoms combine, the idea of relative atomic mass can be extended to compounds and the term relative formula mass , M r , is used. For example, the relative formula mass of copper(II) sulfate, CuSO 4 is: (1 × 63.5) + (1 × 32) + (4 × 16) = 159.5

Key terms Relative atomic mass is the average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12. Relative isotopic mass is the mass of an atom of an isotope relative to one-twelfth the mass of an atom of carbon-12. Relative formula mass is the sum of the relative atomic masses of all atoms present in its formula.

Study point Relative formula mass used to be called relative molecular mass, but relative molecular mass really refers to compounds containing only molecules.

The mass spectrometer When a mass spectrometer is used to find the relative atomic mass of an element it measures two things:

▪ The mass of each different isotope of the element. ▪ The relative abundance of each isotope of the element. The diagram below shows how a mass spectrometer works.

IONISATION 33 DRAFT ACCELERATION Electromagnet To vacuum pump DEFLECTION DETECTION Vaporised sample Ampli er Chart recorder Knowledge check Calculate the relative formula mass for (a) Ca(OH) 2 (b) Na 2 S 2 O 3 .5H 2 O 1