Chemistry · Solutions · NEET
Mole fraction of a component = (moles of that component) / (total moles of all components). For a mixture of A and B: x_A = n_A / (n_A + n_B) and x_B = n_B / (n_A + n_B). The key rule NTA tests: the NUMBER on top must be the same component whose mole fraction you are finding. If you write n_A on top, that is x_A, NOT x_B.
No. Mole fraction has NO unit. You are dividing moles by moles, so the units cancel out. It is just a pure number between 0 and 1. This is why NEET uses it in Raoult's law and Henry's law — it is a clean ratio with no unit attached.
Because every mole in the solution is counted once. If you add x_A + x_B + x_C + ... you are adding (n_A + n_B + n_C + ...) / (total moles), and the top now equals the total moles. So it becomes total/total = 1. This is a fast trick: if you know x_solute, then x_solvent = 1 minus x_solute. You do not need to recalculate.
No. Mole fraction depends only on moles (which come from mass, not volume). Mass does not change when temperature changes. Volume does change with temperature — that is why molarity is temperature-dependent but mole fraction, molality, mass % and ppm are NOT. NEET 2017 asked exactly this and the answer was molarity, because only molarity uses volume.
First convert each mass to moles by dividing mass by molar mass. Example: 20% by mass ethylene glycol (C2H6O2, molar mass 62) in water means 20 g glycol and 80 g water in 100 g solution. Moles glycol = 20/62 = 0.322, moles water = 80/18 = 4.44. Then x_glycol = 0.322 / (0.322 + 4.44) = 0.068. Always turn mass into moles FIRST, then apply the ratio.
Mole fraction is the concentration unit used inside two big formula chapters: Raoult's law (vapour pressure = pure vapour pressure x mole fraction) and Henry's law (p = K_H x). If you get the mole fraction wrong, every vapour pressure and deviation question after it will be wrong. Many NEET questions give you moles or a molar ratio and expect you to jump straight to mole fractions.
For a binary mixture of A and B, the number of moles of A and B are n_A and n_B respectively. The mole fraction of B is given as x_B = n_A / (n_A + n_B). Is this statement correct?
Which of the following is dependent on temperature?
In water-saturated air the mole fraction of water vapour is 0.02. If the total pressure of the saturated air is 1.2 atm, the partial pressure of dry air is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the share of total moles taken by one part of the mixture. If a solution has 1 mole of solute and 3 moles of water (4 total), the mole fraction of solute is 1/4 = 0.25.
The symbol is x, with a subscript for the component. x_A means mole fraction of A. Sometimes the solute mole fraction is written x_2 and the solvent x_1.
No. Mole fraction is always between 0 and 1, because one part can never have more moles than the whole. All mole fractions in a mixture add up to exactly 1.
Mole percent is just mole fraction multiplied by 100. If x = 0.25, the mole percent is 25%. They describe the same thing.
Use mole fraction in vapour pressure problems (Raoult's law) and gas solubility (Henry's law), and whenever temperature may change. Use molarity when you need concentration per litre of solution for reactions.
