Butanoic acid or butaneoic acidWatch this thread
However, AQA themselves are not always correct. AQA insist on ethylamine (although aminoethane can gain credit), but the correct IUPAC name for this compound is ethanamine. In AQA's defence, the name ethylamine has been around so long, it's become accepted and IUPAC recognize it as such, but it's not their preferred systematic name.
It applies when you have multiple functional groups that need a number to state where they are.
The position of the OH group in butanol is ambiguous and hence needs a locant, e.g. butan-1-ol.
If you take the -1- out, it becomes butanol which is sensible.
Add a second OH and you get something like butane-1,2-diol.
You should note that I added an 'e'. That's because when you take the locants out, you get butanediol. As opposed to butandiol. I underlined the 'and' as that is how you might put emphasis on the pronunciation, which would be wrong.
The rule basically goes: if you remove the locants and the suffix starts with a vowel, then no 'e' is needed e.g. butan_ol. But is the suffix starts with a consonant then add an 'e', e.g. butane_diol.