Chemistry Unit 1

Unit 1.3 – Chemical calculations

Key Words

Relative atomic mass – is the average mass of the element relative to one-twelfth the mass of one atom of carbon-12

Relative isotopic mass – is the mass of an atom of an isotope relative to one-twelfth the mass of an atom of carbon-12

Relative formula mass – average mass of a molecule relative to one-twelfth the mass of an atom of carbon-12.

One mole – the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12

Avogadro constant – number of atoms per mole

Molar mass – is the mass of one mole of a substance

Stoichiometry – the molar relationship between the amounts of reactants and products in a chemical reaction

Empirical formula – simplest formula showing the simplest whole number ratio of the amounts of elements present

Molecular formula – shows the actual number of atoms of each element present in the molecule.

Molar volume – volume per mole of gas

Mass spectrometer

Atoms and molecules can be deflected by magnetic fields - provided the atom or molecule is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't.

Stage 1: Ionisation

The vaporised sample passes into the ionisation chamber. The particles in the sample (atoms or molecules) are therefore bombarded with a stream of electrons, and some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions.

Stage 2: Acceleration

The ions are accelerated so that they all have the same kinetic energy. 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. The middle slit carries some intermediate voltage. All the ions are accelerated into a finely focused beam.

Stage 3: Deflection

The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.

Stage 4: Detection- by the generation of an electric current

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.

What the mass spectrometer output looks like

The output from the chart recorder is usually simplified into a "stick diagram". This shows the relative current produced by ions of varying mass/charge ratio.

You may find diagrams in which the vertical axis is labelled as either "relative abundance" .The vertical scale is related to the current received by the chart recorder - and so to the number of ions arriving at the detector: the greater the current, the more abundant the ion.

As you will see from the diagram, the commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of…