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If a substance is written just as a symbol, e.g. H or He, this tells you that it is an element (see p 137). You need to look carefully to see how many capital letters there are. For example, Co is the element cobalt but CO is the compound carbon monoxide. By using the periodic table on p 162 you can compare the mass of each atom of any element to that of carbon — which is taken as 12.
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