<P> A triple bond may be expressed with three lines (HC ≡ CH) or three pairs of dots (HC::: CH), and if there may be ambiguity, a single line or pair of dots may be used to indicate a single bond . </P> <P> Molecules with multiple functional groups that are the same may be expressed by enclosing the repeated group in round brackets . For example, isobutane may be written (CH) CH . This condensed structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions (isomers). The formula (CH) CH implies a central carbon atom connected to one hydrogen atom and three CH groups . The same number of atoms of each element (10 hydrogens and 4 carbons, or C H) may be used to make a straight chain molecule, n - butane: CH CH CH CH . </P> <P> In any given chemical compound, the elements always combine in the same proportion with each other . This is the law of constant composition . </P> <P> The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio . For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2: 1 ratio . If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen . This mass proportion will be the same for any water molecule . </P>

What does the chemical formula for acid often begin with