Molarity (M) tells you how many moles of solute are dissolved in 1 litre of solution. The formula is M = moles of solute / volume of solution in litres. Memory hook: the "L" in molarity stands for the Litre of the whole solution, and because volume changes when it gets hot or cold, molarity is the concentration unit that depends on temperature.
Molarity equals moles of solute divided by the volume of the whole solution in litres. Because the volume of a solution expands when heated, molarity is the concentration unit that changes with temperature.
Your doubts, answered
What exactly is molarity in simple words?
Molarity (symbol M) is the number of moles of solute dissolved in 1 litre of solution. If a bottle says 0.25 M NaOH, it means 0.25 mole of NaOH is dissolved to make 1 litre of solution. It is a way to say how strong or how dilute a solution is. NEET loves this unit because it is used in titrations, dilution, and colligative property questions.
What is the molarity formula?
Molarity = moles of solute divided by volume of solution in litres. In symbols: M = n / V, where n is moles of solute and V is volume of the whole solution in litres. If the volume is given in mL, first divide by 1000 to change it to litres. The unit of molarity is mol/L, also written as M.
Do I use the volume of the solvent or the whole solution?
Always use the volume of the whole solution, not just the solvent. This is the number one mistake students make. When you dissolve solute in water, the total volume changes a little, so we measure the final volume of the mixed solution. Molality (m) is the one that uses solvent mass, not molarity.
How do I get moles when the question gives grams?
Use moles = mass in grams / molar mass. For example, 5 g of NaOH has molar mass 40 g/mol, so moles = 5 / 40 = 0.125 mol. Then put this into the molarity formula. Almost every NEET molarity problem starts with this grams-to-moles step, so practise it until it is automatic.
Why does molarity change with temperature but molality does not?
Molarity uses volume, and volume expands when heated and shrinks when cooled. So the same solution has a slightly different molarity at 25 C and at 50 C. Molality uses mass, and mass never changes with temperature. This is why NEET asks 'which unit depends on temperature?' and the answer is molarity.
How do I convert molarity to molality?
You need the density of the solution. Take 1 litre of solution, find its mass using mass = density x 1000. Find the mass of solute from the moles. Subtract to get the mass of solvent, change it to kg, then molality = moles of solute / kg of solvent. This appears often in NEET, so learn the density step well.
⚠️ The NEET trap ✗ Students think molarity stays the same when the solution is heated, so they mark molality or mole fraction as the temperature-dependent unit. ✓ Molarity is defined per litre of solution, and volume expands with heat, so molarity is the unit that changes with temperature. Molality, mole fraction, and mass percentage all use mass, so they stay fixed. 🧠 If a unit has 'volume' in its definition, it feels the heat. Molarity uses volume of solution, so molarity changes with temperature.
Real NEET questions
NEET 2017
Which of the following is dependent on temperature?
A · Molality
B · Molarity ✓
C · Mole fraction
D · Weight percentage
Solution: Molarity is defined as moles of solute per litre of SOLUTION, so it depends on volume. Volume expands or shrinks when temperature changes, therefore molarity changes with temperature. Molality, mole fraction, and weight percentage are all based on mass or moles, which do not change with temperature. So the only temperature-dependent unit here is molarity (option B).
NEET 2026
Find the molarity of a solution containing 5 g of NaOH (molar mass 40 g/mol) in 450 mL of solution at 298 K.
A · 0.278 M ✓
B · 0.556 M
C · 0.125 M
D · 0.450 M
Solution: Step 1: moles of NaOH = mass / molar mass = 5 / 40 = 0.125 mol. Step 2: change volume to litres = 450 / 1000 = 0.45 L. Step 3: Molarity = moles / volume in litres = 0.125 / 0.45 = 0.278 M. So the answer is 0.278 M (option A). This exact statement appeared as a correct statement in NEET 2026.
NEET 2019 Odisha
The density of a 2 M aqueous solution of NaOH is 1.28 g/cm^3. Find the molality of the solution. (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 litre (1000 mL) of the 2 M solution, which contains 2 moles of NaOH. Mass of solution = density x volume = 1.28 x 1000 = 1280 g. Mass of solute = 2 x 40 = 80 g. Mass of solvent (water) = 1280 - 80 = 1200 g = 1.2 kg. Molality = moles of solute / kg of solvent = 2 / 1.2 = 1.67 m (option C). This shows how molarity plus density gives molality.
Solved Solutions NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The unit of molarity is mol/L, also written as mol L^-1 or simply M. For example, 0.25 M means 0.25 mole of solute per litre of solution.
Is molarity the same as concentration?
Molarity is one type of concentration. Concentration is a general word for how much solute is present; molarity is the specific measure of moles of solute per litre of solution.
Does molarity increase or decrease when you heat a solution?
Heating makes the solution expand, so its volume increases. Since molarity = moles / volume, a bigger volume means molarity decreases slightly when the solution is heated.
How is molarity different from molality?
Molarity uses the volume of the solution in litres, while molality uses the mass of the solvent in kilograms. Because volume changes with temperature but mass does not, molarity is temperature-dependent and molality is not.
Can molarity be greater than 1?
Yes. Molarity just counts moles per litre, so concentrated solutions can be several molar. For example, concentrated HCl is about 12 M.
