Chemistry · Solutions · NEET
Molality is a way to measure how strong a solution is. It counts the moles of solute (the thing you dissolve) present in 1 kilogram of the solvent (the thing that does the dissolving). The symbol is a small letter m. So '2 m' means 2 moles of solute in 1 kg of solvent. Note: it is the mass of the SOLVENT only, not the whole solution.
Molality (m) = moles of solute / mass of solvent (in kg). To get moles of solute, divide the given mass of solute by its molar mass. Then divide by the solvent mass converted to kilograms. Example: 2 moles of solute in 500 g (0.5 kg) of solvent gives m = 2 / 0.5 = 4 m.
Only the mass of the SOLVENT. This is the most common mistake. Molality uses solvent mass in kg, not solution mass. So if a problem gives you the total solution mass, you must subtract the solute mass first: mass of solvent = mass of solution minus mass of solute.
Molality is built only from mass (moles of solute and kg of solvent). Mass does not change when you heat or cool the solution. Volume, on the other hand, expands when heated. Since molality uses no volume, it stays the same at every temperature. This is why NEET prefers molality for colligative property formulas like ΔTf = Kf m and ΔTb = Kb m.
Molarity (M) = moles of solute per litre of SOLUTION (uses volume). Molality (m) = moles of solute per kg of SOLVENT (uses mass). Molarity changes with temperature; molality does not. Easy way to remember: molaLiTY = Litre (volume), molaLIty = mass of solvent.
Take exactly 1 litre (1000 mL) of the solution. Moles of solute = the molarity value. Mass of solution = 1000 x density (in g/mL). Mass of solute = moles x molar mass. Mass of solvent = mass of solution minus mass of solute, then convert to kg. Finally molality = moles of solute / mass of solvent in kg. This exact steps chain is tested almost every year in NEET.
A one molal (1 m) solution has exactly 1 mole of solute dissolved in 1 kg (1000 g) of solvent. If it contains 0.5 mole of solute, then the mass of solvent must be 0.5 kg = 500 g, because 1 = 0.5 / mass in kg.
In a one molal solution that contains 0.5 mole of a solute, there is:
The density of a 1 M solution of a compound 'X' is 1.25 g/mL. The molality of the solution is (molar mass of X = 85 g/mol):
The density of a 2 M aqueous solution of NaOH is 1.28 g/cm^3. The molality of the solution is (molar mass of NaOH = 40 g/mol):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The unit of molality is mol/kg, often written as m (a small letter). It means moles of solute per kilogram of solvent.
No. Molarity uses litres of solution (volume), while molality uses kilograms of solvent (mass). They give different numbers for the same solution, and only molarity changes with temperature.
Because those experiments involve heating and cooling. Molality does not change with temperature, so it gives reliable, constant values in ΔTf = Kf m and ΔTb = Kb m.
Always the mass of the solvent alone, in kilograms. If a question gives total solution mass, subtract the solute mass first to get solvent mass.
Divide the mass of the solute (in grams) by its molar mass (in g/mol). For example, 80 g of NaOH with molar mass 40 g/mol equals 80/40 = 2 moles.
