I wouldn't include charges in writing a formula, just like I wouldn't write sodium hydroxide as Na
+OH
- rather than NaOH.
I would give charges if I was howing a structure where I was noting the ionic rather than covalent bonding... and I would
never put something like
CH3(CH2)12-C(=O)-O-Na
as it implies the existence of covalent bonding between the O and the Na, and this is untrue.
I would say that any of
- CH3(CH2)12COONa
- CH3(CH2)12CO2Na
- CH3(CH2)12COO- Na+
- CH3(CH2)12CO2- Na+
and any similar answers should be accepted for a question like this.