molarity
/ moh-LAR-ih-tee /
Imagine you want a recipe that counts particles, not just weight, because in chemistry it is the number of molecules that react, not their heft. Molarity is the chemist's everyday answer: it tells you how many moles of solute are dissolved in each litre of solution. A 1-molar (1 M) solution holds one mole of solute — about 6 x 10^23 particles — in every litre.
In symbols, molarity = moles of solute / litres of solution, with units of mol/L, written M and read "molar." To mix a 1 M sugar solution you weigh out one mole of sugar, drop it in a flask, and top up with water to the one-litre mark. Note the volume is the volume of the finished solution, not of the water you started with.
Molarity is popular because the volume of a liquid is so easy to measure — you just read a flask or a cylinder. Its one weakness is that liquids expand and contract with temperature, so the same solution can have a slightly different molarity when warm versus cold. When that matters, chemists switch to molality, which uses mass and ignores temperature.
Dissolve 58.5 g of table salt (one mole of NaCl) in water and make up to exactly 1 litre — you have a 1 M salt solution.
Molarity = moles of solute per litre of finished solution.
Don't confuse molarity (mol per litre of solution) with molality (mol per kilogram of solvent). The names look almost identical but the denominators differ — volume of solution versus mass of solvent — and so do the symbols, M versus m.