# WJEC AS Chemistry- Chemical Calculations

complete set of notes from a new WJEC textbook on chemical calculations

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• Created by: Abbie_Mac
• Created on: 10-04-13 14:47

## Pages in this set

### Page 1

Chemical Calculations
Atomic and Molecular Masses
Most elements exist naturally as one or two isotopes. The mass of an element depends
on the relative abundance of its isotopes present in the sample. We use the average
mass of all the atoms, and this is known as relative atomic mass.

Relative…

### Page 2

The Mass Spectrometer
The mass spectrometer measures the masses of atoms and molecules; it can also be
used to measure the relative abundance of different isotopes and to predict complex
molecular structure.

1. Vaporisation: Sample is in vapour form and is released into ionization chamber.

2. Ionisation: Particles are…

### Page 3

Calculations using Mass Spectrometer

The relative atomic mass can be calculated using this formula:

Relative Atomic Mass=

(percentage abundance x mass) + (percentage abundance x mass) ...

100

i.e. RAM = (90 x 20 + 10 x 22)/100 = 20.2

Other uses of mass spectrometry include:

Identifying unknown compounds
Identifying…

### Page 4

The Mass Spectrum of Chlorine

Chlorine is made up of two isotopes, Cl35 and Cl37. However, chlorine gas consists of
molecules not individual atoms.

When chlorine is passed into the ionisation chamber, an electron is knocked off the
molecule to give a molecular ion, Cl2+. These ions won't be particularly…

### Page 5

And the ratio of peaks C:D:E is 9:6:1

Empirical and Molecular Formula
Empirical formula is the simplest formula showing the simplest whole number ratio of the
amount of element present in a molecule.

Molecular formula shows the actual number of atoms of each element in the molecule. It
is a…

### Page 6

Reactions of Solids and Gases
An equation tells us not only what substances react together but also what amounts of
substances react together. The number of moles of a substance, as given by balanced
equation, is called stoichiometric amounts.

The number of moles of solids can be calculated from their…

### Page 7

Volume 02 = n x 22.4 = 0.015 x 22.4 = 0.336 dm3 or 336 cm3

Atom Economy= a value obtained from the chemical equation for the reaction.

Mass of required product/total mass of reactants x 100

Percentage Yield= calculated form the mass of product actually obtained by experiment.

Mass…