WJEC AS Chemistry- Chemical Calculations

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

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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 Atomic Mass: the average mass of one atom of the element relative top one
twelfth of the mass of the Carbon-12.
Relative Atomic mass has no units. If the mass of a particular isotope is referred to, then
relative isotopic mass is used.
Relative Isotopic Mass: the mass of an atom of an isotope relative to one-twelfth the
mass of an atom of Carbon-12.
The mass of a molecule is measured as relative molecular mass, and is the sum of the
relative atomic masses of all the atoms present in the molecule. For ionic compounds, the
formula represents a formula unit, rather than molecules of a compound. May refer to the
relative formula mass instead of relative molecular mass.
Relative Molecular Mass: the average mass of one molecule relative to one twelfth the
mass of an atom of Carbon-12.
Amount of Substance
In chemical reactions the atoms that make up the reactants re arrange to form the
products. For all the reactants to change into products the correct quantities must be
One Mole: the amount of any substance that contains the same number of particles as
there are atoms in exactly 12g of Carbon-12.
This number is 6.02 x 1023, and is called the Avogadro Constant.
The mass per mole of an element or compound is called the molar mass, M, and has units
g mol-1.

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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) ...
i.e. RAM = (90 x 20 + 10 x 22)/100 = 20.2
Other uses of mass spectrometry include:
Identifying unknown compounds
Identifying trace compounds in forensic science
Analysing molecules in space
Sometimes, the molecules can't cope with the extreme conditions in the mass
spectrometer and they can break apart. This is called fragmentation.…read more

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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 stable, and some will
fall apart to give a chlorine atom and a Cl+ ion. (This is fragmentation.)
So the first peak is caused by 35Cl+ and the second by 37Cl+.…read more

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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 simple multiple of the empirical formula. Usually the relative molecular mass is needed
to determine the molecular formula.
Example: a compound of Carbon, Hydrogen and Oxygen has a relative molecular mass of
180.…read more

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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 masses but the numbers of
moles of gases are calculated from their volumes using the molar gas volume.…read more

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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.…read more


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