Chemistry · Some Basic Concepts Of Chemistry · NEET
Molarity uses SOLUTION. Molarity (M) = moles of solute divided by volume of the whole SOLUTION in litres. Solution means solute plus solvent mixed together. Molality (m) uses only the SOLVENT. Molality = moles of solute divided by mass of the SOLVENT in kilograms. So one uses the full mixture (volume), the other uses just the liquid you dissolved into (mass). Getting this one line right fixes most NEET errors.
Molarity is per litre of solution, and volume expands when heated and shrinks when cooled. So when temperature goes up, the same solution takes more space, the volume increases, and molarity goes DOWN. Molality is per kilogram of solvent. Mass never changes when you heat or cool something. So molality stays the SAME at any temperature. This is why molality is preferred for experiments where temperature changes. NEET has asked this exact idea directly.
Molarity unit is mol/L, written as M (capital M) and read as 'molar'. Molality unit is mol/kg, written as m (small m) and read as 'molal'. Careful: capital M means molarity, small m means molality. They look almost the same but mean different things, so read the question letter carefully in the exam.
Follow a fixed trick used in NEET. Step 1: take 1 litre (1000 mL) of solution. Step 2: mass of solution = 1000 x density (in g). Step 3: mass of solute = molarity x molar mass. Step 4: mass of solvent = mass of solution minus mass of solute, then change grams to kg. Step 5: molality = moles of solute divided by mass of solvent in kg. Density is the bridge that connects volume to mass, which is why they always give it.
For a dilute water solution they are almost equal because 1 litre of water is about 1 kg. But for a concentrated solution, molality is usually a little larger than molarity, because the mass of solvent (in kg) is smaller than the volume of solution (in L) after you remove the solute. In every NEET density problem you have seen, the molality came out different from the molarity, so never assume they are equal.
No. Mole fraction, mass percent (w/w %), and molality are all based on MASS, and mass does not change with temperature. Only molarity is based on VOLUME, so only molarity changes with temperature. NEET loves to ask 'which concentration term depends on temperature?' and the answer is molarity.
Which of the following concentration terms 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 NaOH solution is 1.28 g/cm3. 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.
Molarity = moles of solute per litre of solution (volume based). Molality = moles of solute per kilogram of solvent (mass based). Molarity changes with temperature, molality does not.
Molality uses mass of solvent, which never changes with temperature. So molality gives the same value whether the lab is hot or cold. This makes it more reliable for properties that depend on temperature.
Molarity uses capital M (mol/L). Molality uses small m (mol/kg). Read the letter carefully in NEET questions because they look similar.
Yes. Density connects volume and mass. Take 1 litre of solution, find its mass using density, subtract the solute mass to get the solvent mass in kg, then divide moles by that mass.
For very dilute water solutions they are almost equal, because 1 litre of water is nearly 1 kg. For concentrated solutions they differ, and molality is usually a bit higher.
