A-Level备考：原子分子的计算复习

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今天给同学们分享下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:

From this data the Relative Atomic Mass can be found.

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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