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

1.3 Chemical calculations

1.3

You should be able to demonstrate and apply knowledge and understanding of: ▪ The various relative mass terms (atomic, isotopic, formula, molecular). ▪ The principles of the mass spectrometer and its use in determining relative atomic mass and relative abundance of isotopes. ▪ Simple mass spectra, for example that of chlorine gas. ▪ How empirical and molecular formulae can be determined from given data. ▪ The relationship between the Avogadro constant, the mole and molar mass. ▪ The relationship between grams and moles. ▪ The concept of concentration and its expression in terms of grams or moles per unit volume (including solubility). ▪ Molar volume and correction due to changes in temperature and pressure. ▪ The ideal gas equation ( pV = nRT ). ▪ The concept of stoichiometry and its use in calculating reacting quantities, including in acid–base titrations. ▪ The concepts of atom economy and percentage yield. ▪ How to estimate the percentage error in a measurement and use this to express numeric answers to a sensible number of significant figures. Chemists are interested in how atoms react together. They want to know about the qualitative and quantitative aspects of chemical reactions. Very often chemists want to measure out exact quantities of substances that will react together – especially in industry where adding too much of a reagent will result in an unnecessary cost or may contaminate the product. What is really useful is to be able to work out these quantities on the basis of the number of atoms (or molecules) of substances that react together. This topic shows how this can be done by using the concept of the mole and balanced chemical equations.

Topic contents 33 Relative mass terms 33 The mass spectrometer 36 Amount of substance 37 Calculating reacting masses

39 Volumes of gases 42 Concentrations of solutions

42 Acid–base titrations 44 Atom economy and percentage yield 45 Errors or uncertainties 47 Test yourself

Maths skills • Use appropriate units in calculations. • Substitute numerical values into algebraic equations. • Change the subject of an equation. • Use standard and ordinary form, decimal places and significant figures. 32 DRAFT • Select values and find arithmetic means. • Identify uncertainties in measurements. • Use ratios, fractions and percentages.