molarity
/ moh-LAR-ih-tee /
Imagine you want to count particles, not weigh them, but you scoop your solution by volume with a flask. Molarity is the bridge: it tells you how many moles of solute sit in each litre of solution.
In symbols, molarity (unit mol/L, often written M) equals moles of solute divided by litres of solution — note, the whole solution, not just the solvent. A '2 M' label means two moles dissolved and topped up to a final volume of one litre.
Molarity is the chemist's favourite everyday unit because reactions count in moles and labware measures volume, so it links recipes to reaction equations directly. Its one weakness: liquids expand and contract with temperature, so a litre is not perfectly fixed, and molarity drifts slightly when a solution is warmed or cooled.
Dissolving 58.4 g of table salt (one mole of NaCl) in water and making it up to exactly 1 litre in a volumetric flask gives a 1.0 mol/L (1 M) solution.
One mole of solute brought to one litre of solution equals 1 M.
Molarity uses litres of solution, while molality uses kilograms of solvent. Mixing them up is one of the most common beginner errors; molality is the one that ignores temperature.