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Relative Atomic Mass

The mass of one atom is so small, it would be silly to use it in calculations. So instead, we use the relative atomic mass of different elements (which we can find on the periodic table).


An atom of carbon-12 (mass of 12) is used as the standard form. It has a 'mass' of exactly 12 units (thanks to its 6 protons, and 6 neutrons). All other atoms are compared to this standard. The easiest example is hydrogen, which has a relative atomic mass (RAM) of 1 - one-tweflth of a C-12 atom.

The RAM is usually the mass of the element's most common isotope, as it is an average of the ratio of any naturally occurring isotopes, which is why an atom of chlorine has a RAM of 35.5 (even though a proton and neutron have a mass of 1).

Relative Formula Mass

Using the RAM of atoms we can work out the relative formula mass (RFM) of compounds. It is as simple as adding up the different RAMs to work out the RFM.

For example:

NaCl = 23 (Na) + 35.5 (Cl) = 58.5

Atomic Percentage

Using the RFM we can work out the percentage, by mass, of specific atoms in a compound.
Question: What percentage of the mass of calcium oxide is calcium?Answer:

  • CaO has a RFM of 40 + 16 = 56
  • So in 40g of CaO, there are 40g of calcium (as RAM is 40)
  • The fraction of Ca is 40/56 = 0.7142
  • The percentage of Ca is 71.4%


It is important to remember that what makes an element an element is the number of protons in the nucleus of its atoms. We've seen what happens when you change the number of electrons (ions), but the number of neutrons can also change - giving rise to isotopes.

Isotopes will always have the same atomic number, but the mass number will vary.


The relative atomic mass (RAM) or relative formula mass (RFM) of a substance in grams, is a mole of that substance.

So, for example:

  • a mole of carbon would be 12g as it has a RAM of 12
  • a mole of water would be 18g as it has a RFM of 18 (16+1+1)

Moles Infographic

Empirical Formula


Below is a helpful video that explains the steps involved.

Reacting Masses


Using balanced equations we can show that if we use the RFM of each part of the equation to be the mass of the molecules, that in actual fact the large numbers in the equation (e.g. 2HCl) do not mean the number of molecules, but actually - the number of moles involved.

In this reaction below, we can see that for every 1 mole of hydrogen, we need 1 mole of chlorine. Together this will make 2 moles of hydrogen chloride.

Theoretical Yield


Below is a helpful video that explains the steps involved.

Percentage Yield

To work out the percentage yield, you simply divide the actual yield, but the theoretical yield (and times it by 100)!

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Reversible Reactions

Most of the reactions you may have seen on this website only show a reaction progressing from left to right (reactants making products), but actually in some reactions the products can react back to make the reactants. This is called a reversible reaction.


E-numbers are additives that are added to food, and the number relates to different types of chemical (colour, flavour etc). One way to test for these additives is to use paper chromatography. This will separate the mixture out into its components based on the chemicals' solubility.

Gas Chromatography

This technique is used to separate compounds that are easily vaporised (turned into a gas).

A carrier gas is used to move the vapour through the column (like the solvent was used to move the inks in paper chromatography).

Instrumental Methods

Industry need quick and accurate measurements to analyse their products, and they use modern instrumental methods to do this.

Advantages over old methods

  • highly accurate and sensitive
  • very quick
  • small samples can be analysed


  • expensive
  • specialist training needed
  • data must be compared with known values to make sense

Mass Spectrometry


A mass spectrometer gives an accurate reading of a compounds RFM. It gives a graph result with a series of peaks, and the one with the largest mass is the molecular ion peak. This peak tells you the mass of the compound, and really represents an ion of the compound (minus an electron) - but because the mass of one electron is so small it can be ignored.

Got to love a bad chemistry joke...

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Page last updated: 16/04/2017