$C = \dfrac{n}{V}$ — C equals n over V — is the chemist's headcount: how many moles of solute swim in each litre of solution. Twice the solute in the same flask, twice the concentration; the same solute in twice the water, half. The unit mol/L says exactly what the formula does — moles upstairs, litres downstairs.

One warning before the first flask: $V$ is measured in litres, and real glassware is graduated in millilitres. Convert before the socket — $250\ \mathrm{mL} = 0.250\ \mathrm{L}$, milli means a thousandth — or your concentration lands a thousand times off. That ×1000 slip is this chapter's most faithful companion; meet it here, on your terms.