Notesale: Turn your study into money

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Chapter 1

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Define the following terms: relative atomic mass;
relative isotopic mass; relative molecular mass;
relative formula mass
Define the mole in terms of the Avogadro Constant
Interpret mass spectra of elements and molecules
in terms of isotopic abundance and molecular
fragmentation
Define the terms empirical and molecular formula
Calculate the relative atomic mass of an element
from the relative abundances of its isotopes or
mass its mass spectrum

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Calculate the empirical and molecular
formula of a compound from combustion
data or composition by mass
Do chemical calculations using reacting
masses of substances and volumes of
gases(including mole)

The relative masses of atoms, molecules and ions are obtained by
comparison with the mass of the atom of the carbon-12 isotope
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of atoms of Y X relative isotopic mass of Y] + [no
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The mass of one mole of atoms of an element
= relative atomic mass of the element in grams
= Ar
The number of moles of atoms in a sample of an element = Mass in grams of the element
Molar mass of the element

The number of particles in one mole of any substance is a constant known as the
Avogadro constant (L)
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02 X 1023 mol -1

The number of particles in a sample = no
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At s
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p
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4dm3
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Volume of gas = Mole X Molar volume

The basic principle
If something is moving and you subject it to a sideways force, instead of moving in a
straight line, it will move in a curve - deflected out of its original path by the
sideways force
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All you've got is a jet of water from a hose-pipe that you can squirt at it
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But suppose instead, you tried to deflect a table tennis ball travelling at the same
speed as the cannonball using the same jet of water
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The amount of deflection you will get for a given sideways force depends on the
mass of the ball
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The less the deflection, the heavier the ball
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Understanding what's going on
The need for a vacuum
It's important that the ions produced in the ionisation chamber have a free
run through the machine without hitting air molecules
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The electrically heated metal coil gives off electrons which are attracted
to the electron trap which is a positively charged plate
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Most of the positive ions formed will carry a charge of +1 because it is
much more difficult to remove further electrons from an already positive ion
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Acceleration
The positive ions are repelled away from the very positive ionisation chamber
and pass through three slits, the final one of which is at 0 volts
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All the ions are accelerated into a finely focused beam
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The
amount of deflection depends on:
the mass of the ion
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the charge on the ion
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Detection
The ion detector detects
(a) Where the ions appear
(b) How many ions appear
This data is then fed into a computer which then prints a mass spectrum of the
elements
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In practice, the ion detector is kept in a fixed position
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What the mass spectrometer output looks like
The output from the chart recorder is usually simplified into a "stick diagram"
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The stick diagram for molybdenum looks lilke this:

 As you will see from the diagram, the commonest ion has a mass/charge ratio of 98
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That means that molybdenum consists of 7 different isotopes
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