Chemistry · Solutions · NEET
Molarity means moles of solute in 1 litre of solution. So if you pick exactly 1 litre, the number of moles is simply equal to the molarity. This makes the numbers easy. For a 2 M solution, 1 litre has 2 moles. You are free to choose any amount, but 1 litre is the smart choice because it removes one unknown.
Density tells you how heavy each mL is. So mass of solution = volume x density. For 1 litre (1000 mL) the mass = 1000 x density. Example: density 1.28 g/mL gives 1000 x 1.28 = 1280 g of solution. Important: the density must be in g/mL (same as g/cm3), and use 1000 mL for 1 litre.
The solution is made of solute + solvent. So mass of solvent = mass of solution - mass of solute. Find the solute mass using moles x molar mass. Then subtract it from the total solution mass. This leftover is the water. Convert it to kilograms by dividing by 1000, because molality uses kg.
Molality (m) = moles of solute divided by mass of solvent in kilograms. In these problems the moles of solute equal the molarity (because you took 1 litre). So molality = molarity / (mass of solvent in kg). This is the last step every time.
For water solutions the molality is usually a bit larger than molarity when the solute is heavy, because the solute takes up mass that you remove. The exact answer depends on density and molar mass. Do not guess a shortcut ratio. Always do the full 1-litre steps or you will lose marks on close options.
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):
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):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Molality = (1000 x Molarity) / (1000 x density - Molarity x molar mass of solute). Here density is in g/mL and molar mass in g/mol. This is just the 1-litre method written as one line. It gives the same answer as the step-by-step way.
Yes. Molarity is per litre of solution (a volume), but molality is per kg of solvent (a mass). Density is the bridge that turns the volume of solution into a mass. Without density you cannot find the mass of the solvent.
Use g/mL, which is the same number as g/cm3. Then 1 litre (1000 mL) of solution has mass = 1000 x density in grams. If density is given in kg/L, the number is the same value, just handle units carefully.
Usually yes, because you remove the solute mass from the solution mass, leaving less solvent, which makes the bottom of the fraction smaller. But the exact answer depends on the numbers, so always calculate. Do not assume.
