Every bottle in a chemistry lab introduces itself the same way: a number, and the unit it wears. mol/L is a molar concentration — moles of dissolved stuff per litre of solution, written $C = \dfrac{n}{V}$ and read aloud: C equals n over V. Chemists nickname it molarity, and a 2 mol/L bottle gets called “two molar” — written 2 M for short.

The slash is the tell. mol/L divides a mole count by a volume — that is molarity. A % divides a mass by a mass — that is mass percent, every unit cancelled. And a number in bare grams or bare moles is not a concentration at all — it is an amount, still waiting for its water. Read the unit before the number, and the label can never fool you.