Cambridge International AS Level Chemistry
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Relative atomic mass is the weighted average mass of naturally occurring atoms of an element on a scale where an atom of carbon-12 has a mass of exactly 12 units. Relative molecular mass, relative isotopic mass and relative formula mass are also 12 based on the C scale. The type and relative amount of each isotope in an element can be found by mass spectrometry. The relative atomic mass of an element can be calculated from its mass spectrum.
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One mole of a substance is the amount of substance that has the same number of particles as there are in exactly 12 g of carbon-12. The Avogadro constant is the number of a stated type of particle (atom, ion or molecule) in a mole of those particles. Empirical formulae show the simplest whole number ratio of atoms in a compound.
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Empirical formulae may be calculated using the mass of the elements present and their relative atomic masses or from combustion data. Molecular formulae show the total number of atoms of each element present in one molecule or one formula unit of the compound. The molecular formula may be calculated from the empirical formula if the relative molecular mass is known. The mole concept can be used to calculate: – reacting masses – volumes of gases – volumes and concentrations of solutions. The stoichiometry of a reaction can be obtained from calculations involving reacting masses, gas volumes, and volumes and concentrations of solutions.
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End-of-chapter questions 1 a i What do you understand by the term relative atomic mass? ii A sample of boron was found to have the following % composition by mass:
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10 B (18.7%), 11 B (81.3%) 5 5
Calculate a value for the relative atomic mass of boron. Give your answer to 3 significant figures. b Boron ions, B3+, can be formed by bombarding gaseous boron with high-energy electrons in a mass spectrometer. Deduce the number of electrons in one B3+ ion. c Boron is present in compounds called borates. i Use the Ar values below to calculate the relative molecular mass of iron(III) borate, Fe(BO2)3. (Ar values: Fe = 55.8, B = 10.8, O = 16.0) ii The accurate relative atomic mass of iron, Fe, is 55.8. Explain why the accurate relative atomic mass is not a whole number.
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Total = 6