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A-LEVEL考试:原子分子的计算复习

2017-12-25 15:01

来源:新东方国际学科

作者:天津新东方 刘欣梦

  点击查看>>>2018留学考试备考规划

  今天给同学们分享下刘欣梦老师给大家讲解的A-LEVEL考试中的原子分子的计算复习。

  Part 1 The Mole

  1Relative atomic mass

  The relative atomic mass, Ar, of an elementis the average mass of the naturally occurring isotopes of the element relative to the mass of an atom of carbon-12.

  The relative molecular mass, Mr, is the mass of a molecule relative to the mass of an atom of carbon-12, which is given the exact mass of 12.

  Example: NH3 = 14 + 3x1 = 17.

  The relative formula mass is used for compounds made up of ions.

  2The mole and Avagadros number

  A mole of substance is the amount of substance that has the same number of particles as there are in 12.00g of carbon-12. The particles may be atoms, molecules, ions or even electrons.

  This number of particles is a constant known as Avagadros constant (L), and is approximately 6 x 1023mol-1.

  The mass of one mole of a substance is often referred to as the molar mass (M). The units of molar mass are g mol-1.

  To find the amount of substance present in a given mass, we must divide that mass by the molar mass of the substance.

  For example - if we had 10g of CaCO3:

  M is 40 + 12 + 3x16 = 100g.

  So in 10g of calcium carbonate there is 10/100 mol = 0.1 mol CaCO3.

  To find the mass of a given substance, we multiply the number of moles of the substance by the molar mass.

  3Calculations of mass from stoichiometric equations

  If we know the mass of a reactant, we can find the mass of a product in a chemical equation.

  Consider the formation of ammonia from hydrogen and nitrogen.

  2N2 (56g) + 3H2 (6g) → 4NH3 (34g)

  Hence, for every 6g of hydrogen we make 34g of ammonia.

  Part 2 Mass Spectrometry

  1Determination of Ar and Mr

  An instrument called a mass spectrometer is used to calculate relative atomic mass.

  In a mass spectrometer, atoms are converted to positive ions; these are then deflected by electric and magnetic fields before being detected. The mass of the elements individual isotopes are found as well as their abundance.

  Example of a mass spectrum is shown below:

A-LEVEL考试:原子分子的计算复习

  From this data the Relative Atomic Mass can be found.

A-LEVEL考试:原子分子的计算复习

  Part 3 Empirical and Molecular Formulae

  1Definitions区分经验式和分子式的关系

  The empirical formula of a compound shows the simplest whole-number ratio of the elements present.

  The molecular formula shows the total number of atoms of each element present in a molecule of a compound.

  Example:

  hexane: C6H14

  Empirical Formula: C3H7

  Molecular Formula: C6H14

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