Converting Molarity to Molality Using Density (NEET)
Chemistry · Some Basic Concepts Of Chemistry · NEET
To change molarity into molality, always start with exactly 1 litre (1000 mL) of solution. Multiply 1000 by the density to get the mass of the whole solution, subtract the mass of the solute, and that leaves the mass of the solvent in grams. Then molality = moles of solute divided by solvent mass in kg. Memory hook: "1 litre first, weigh it, take out the solute, what is left is the solvent."
Three steps: take 1 L of solution to get moles and mass, subtract the solute mass to find solvent mass, then divide moles by solvent mass in kg to get molality.
Your doubts, answered
Why do we always start by taking 1 litre (1000 mL) of solution?
Because molarity is defined per 1 litre of solution. If the solution is 2 M, then 1 litre contains exactly 2 moles of solute. Starting with 1 litre gives you the moles for free, with no extra work. You can pick any volume, but 1 litre makes the numbers cleanest for NEET.
What is the difference between mass of solution and mass of solvent?
Mass of solution = solvent + solute together (the whole liquid you weigh). Mass of solvent = only the water (or liquid that dissolves things). To get the solvent, you must SUBTRACT the solute mass from the solution mass. Forgetting this subtraction is the most common NEET mistake.
How do I get the mass of the solution from density?
Mass = density x volume. Take 1000 mL of solution and multiply by the density (in g/mL). Example: density 1.25 g/mL gives mass = 1000 x 1.25 = 1250 g. Density is the bridge that turns volume into mass.
What is the full formula for molarity to molality?
molality = (1000 x M) / (1000 x d - M x Molar mass). Here M is molarity, d is density in g/mL, and Molar mass is in g/mol. But it is safer to do it step by step in the exam than to memorise this, because you can forget the units.
Why is molality per kg but molarity per litre?
Molality uses mass of SOLVENT in kilograms, so you divide moles by kg (convert grams to kg by dividing by 1000). Molarity uses VOLUME of solution in litres. This is why the density is needed: it connects volume to mass so you can move between them.
⚠️ The NEET trap ✗ Using the mass of the SOLUTION (e.g. 1250 g) as the solvent mass and getting molality = 1/1.25 = 0.80 m. ✓ Subtract the solute mass first: solvent = 1250 - 85 = 1165 g = 1.165 kg, so molality = 1/1.165 = 0.858 m. 🧠 NTA loves testing if you forgot to remove the solute. Solution mass is NOT solvent mass. Always subtract the dissolved substance before dividing.
Real NEET questions
NEET 2023 Phase 2
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)
A · 1.165 m
B · 0.858 m ✓
C · 0.705 m
D · 1.208 m
Solution: Take 1 L (1000 mL) of solution. Since it is 1 M, it holds 1 mol of X. Mass of solution = 1000 x 1.25 = 1250 g. Mass of solute = 1 x 85 = 85 g. Mass of solvent = 1250 - 85 = 1165 g = 1.165 kg. Molality = 1 / 1.165 = 0.858 m. Answer (B).
NEET 2019 Odisha
The density of a 2 M aqueous solution of NaOH is 1.28 g/cm3. The molality of the solution is (molar mass of NaOH = 40 g/mol)
A · 1.20 m
B · 1.56 m
C · 1.67 m ✓
D · 1.32 m
Solution: Take 1 L (1000 cm3) of solution containing 2 mol NaOH. Mass of solution = 1000 x 1.28 = 1280 g. Mass of solute = 2 x 40 = 80 g. Mass of solvent = 1280 - 80 = 1200 g = 1.2 kg. Molality = 2 / 1.2 = 1.67 m. Answer (C).
Solved Some Basic Concepts Of Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is molality always smaller than molarity for water solutions?
Usually yes when density is greater than 1 g/mL, because the solvent mass in kg is a little less than the volume in litres. But this is not a rule to depend on; always calculate. For dilute solutions the two values are very close.
What units must density be in for this conversion?
Use grams per millilitre (g/mL), which is the same as g/cm3. So 1.28 g/cm3 = 1.28 g/mL. Then 1000 mL x density gives grams directly.
Why does NEET like this type of question?
It checks three skills at once: the definition of molarity, using density to get mass, and the definition of molality. One small slip (like forgetting to subtract solute) gives a wrong option that NTA puts in the choices as a trap.
Do I need the molar mass of the solute?
Yes. You need it to find the mass of the solute (moles x molar mass) so you can subtract it and get the solvent mass. That is why every such question gives you the molar mass.
