How can I draw possible structures for an ester that has a molecular ion with an m/z value of 74?
You subtract the masses of the atoms you know are present, and then you work out all possible combinations of atoms that could give you the remaining mass.
You know that the compound is an ester,
The possibilities are:
The corresponding formulas are
This seems to be in the context of mass spectroscopy, so I'm assuming that you're trying to identify an unknown compound by looking at its spectrum and knowing that it's an ester ahead of time from, say, IR spectroscopy.
A molecular mass (MM) of
So this suggests that your compound could be methyl acetate.
Another isomeric possibility, if you move the left methyl group to the rightmost connection:
which is ethyl formate.
It is unclear which one is more likely, but if you also have the IR spectrum, that should help. An aldehyde bending vibration peak on there is somewhere near
Fortunately, methyl acetate would have one well-defined peak, whereas ethyl formate would give you two indistinct, overlapping peaks in that vicinity---the same carbonyl is shared in an aldehyde and ester group. That aldehyde+ester peak would be wider and less well-defined.
Also, ethyl formate would give a very obvious wide peak near
Here are the IR spectra of each one for you to compare.