0.o the shorthand for concentration is just [...]
[NaCl] = Concentration of NaCl
0.o(lol) how about using it in cases such as "A standard solution is a prepared solution of known conc..."
Std. solution=[solution] known.
I SWEARS IT'S LEGAL 0.o
But anyway, I guess it's up to your common sense to decide what's acceptable and what's not, and that's usually in terms of what's often used and what's not. Anything which is 'maths notation' should usually be avoided in short answers (although there are exceptions I'd say). A full proof way is to define your abbreviations and acronyms as you go.
e.g. DNA is composed of four nucleotides, adenine (A), thymine (T), guanine (G) and cytosine (C). Only A and T can bind together, and G with C in the same manner.
The other foolproof way is just not to be lazy and don't abbreviate, especially if you're in doubt.