Chemistry · Solutions · NEET
Molarity (M) = moles of solute divided by litres of SOLUTION (solute + solvent together). Molality (m) = moles of solute divided by kilograms of SOLVENT only. So molarity is per unit VOLUME of the whole solution, while molality is per unit MASS of just the solvent. Same solute amount on top, but the bottom (denominator) is different.
Molarity has volume (litres of solution) in the denominator. When you heat a solution, it expands, so its volume increases; on cooling it shrinks. Since moles of solute stay the same but volume changes, molarity changes. Molality has mass (kg of solvent) in the denominator. Mass does not change when you heat or cool something (no atoms are added or removed). So molality is fixed at every temperature. NCERT says it plainly: 'molality is independent of temperature, whereas molarity is a function of temperature. This is because volume depends on temperature and the mass does not.'
This is the most common mix-up. MolaRity uses the total SOLUTION volume (both letters give you 'volume of solution'). MolaLity uses only the SOLVENT mass. A quick trick: molaLity has an 'L' like kiLogram and soLvent, so it is moles per kg of solvent.
Four terms do NOT change with temperature: molality, mole fraction, mass percentage (weight %), and ppm. All of these are based on mass or number of moles only. The ONLY term that changes with temperature is molarity, because it uses volume. NEET has asked exactly this as a single-line question.
Colligative properties (boiling point elevation, freezing point depression) are measured while heating or cooling the solution, so the temperature is not constant. If we used molarity, the volume and therefore the value would keep changing as temperature changes, giving wrong readings. Molality stays the same at all temperatures, so formulas like ΔTb = Kb·m and ΔTf = Kf·m use molality. This is why molality matters a lot for NEET colligative property numericals.
Yes, for very dilute water solutions the density is close to 1 g/mL, so 1 litre of solution weighs about 1 kg and is almost all solvent. Then molarity and molality come out almost equal. But this shortcut is only for dilute water solutions. For concentrated solutions (like 2 M NaOH) you must convert properly using density, and the two values differ a lot.
Which of the following is dependent on temperature?
In a one molal solution that contains 0.5 mole of a solute, there is:
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.
Not always. For dilute water solutions they are nearly equal. For concentrated solutions it depends on density. If the solution is denser than water, molality is usually a bit less than molarity (as in 2 M NaOH -> 1.67 m). But if the solvent is lighter than water or the solution is very concentrated, molality can be larger. Always convert using density; do not assume.
Molarity has units of mol/L (also written M or mol dm^-3). Molality has units of mol/kg (also written m or mol kg^-1). Notice: molarity ends in per litre (volume), molality ends in per kilogram (mass).
Because these experiments change the temperature of the solution. Molarity would change as the volume changes with temperature, giving a moving value. Molality stays constant, so the formulas ΔTf = Kf·m and ΔTb = Kb·m always use molality. This is a common NEET trap: do not plug molarity into these formulas unless told the solution is dilute and aqueous.
Yes. Density lets you find the total mass of the solution from its volume. Then you subtract the mass of solute to get the mass of solvent, and divide moles of solute by that mass in kg. Learn this conversion next in 'How to Convert Molarity and Density to Molality' - it is a frequent NEET numerical.
Yes. Molality, mole fraction, mass percentage and ppm all depend only on mass or number of moles, which do not change with temperature. Only molarity, which uses volume, changes with temperature. NEET often tests this exact fact in one line.
