1Summary: Chapter 6© Cambridge University Press IGCSE Chemistry

Chapter 6SummaryYou should know that:

the atomic masses of atoms have been found using a mass spectrometer!many elements are made up of different isotopes – atoms of the same element with different mass numbers!the masses of atoms are measured relative to a standard – a carbon-12 atom is fi xed as having a mass of 12 exactly!calculation of the average mass of an atom of an element must take into account the different isotopes present!this average mass is known as the relative atomic mass!the relative formula mass of a compound can be calculated by adding up the masses of the atoms or ions in the formula!relative formula masses are useful for calculating the percentage by mass of an element in a compound!experiments show that compounds are made from elements combining in fi xed ratios by mass.!

! the mole is the unit used in chemistry to express the amount of a substance taking part in a reactionone mole of any substance contains the same number of constituent particles per mole (the Avogadro constant) whether they are !atoms, molecules or formula unitsthe idea of the mole can be used to calculate empirical formulae and molecular formulae from data on the masses of elements in !a compoundthe formulae of hydrated salts can be worked out in a similar way!the balanced chemical equation can be used to calculate the reacting masses of substances and the amount of product formed!one mole of any gas has a volume of approximately 24 dm! 3 at room temperature and pressurefor reactions involving only gases, the ratio of the reactant volume and product volume is the same as the ratio of the number of !moles in the balanced equationthe ‘concentration’ of a solution can be expressed in terms of the mass of solute dissolved (in grams) – this gives the mass !concentration in grams per cubic decimetre (g/dm3)the ‘concentration’ of a solution can be more usefully expressed in terms of the amount of solute dissolved (in moles) – this gives !the concentration in moles per cubic decimetre (mol/dm3)the concentration values (mol/dm! 3) are important in calculating the results of titration experimentscalculations using solubilities can be used to predict the mass of crystals formed by cooling a solution.!

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