Physics · Kinetic Theory · NEET
Partial pressure of a gas in a mixture is the pressure that gas would produce if it alone occupied the whole container at the same volume V and temperature T. Each gas ignores the others. For gas 1, P1 = n1 R T / V, where n1 is the number of moles of gas 1. The total pressure you measure with a gauge is the sum of all these partial pressures.
All the gases share the same container, so they all have the same volume V and the same temperature T. In kinetic theory, molecules do not interact (dilute gas), so molecules of gas 1 hit the wall independently of gas 2. Each set of molecules adds its own wall force, so pressures add. Volume does not add because there is only one box. Write it as P_total = P1 + P2 + ... with V and T common to all.
For a mixture, total moles n = n1 + n2 + n3 + ... . Apply PV = nRT to the whole mixture: P V = (n1 + n2 + n3) R T. Split the right side: P V = n1 R T + n2 R T + n3 R T. Divide by V: P = n1RT/V + n2RT/V + n3RT/V = P1 + P2 + P3. So the total pressure is just the sum of the partial pressures. Notice pressure depends only on total moles, not on which gas it is.
Dalton's law is exact only for ideal gases, where molecules have no size and no attraction. It works well for real gases at low pressure and high temperature, because then molecules are far apart and behave nearly ideally. At high pressure or low temperature the gases attract or repel each other, so the law becomes only approximate. For NEET, unless told otherwise, treat all gases as ideal and apply the law directly.
No. Partial pressure depends on the number of moles (number of molecules), not on the mass of one molecule. From P1 = n1RT/V, only n1 matters. A light gas and a heavy gas with the same number of moles at the same V and T give equal partial pressures. This is because pressure comes from how often molecules hit the wall and how fast, and lighter molecules move faster, which balances their smaller mass.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The total pressure of a mixture of non-reacting ideal gases is equal to the sum of the partial pressures of the individual gases at the same volume and temperature: P = P1 + P2 + P3 + ... .
P1 = n1 R T / V, where n1 is moles of that gas, R is the universal gas constant, T is temperature in kelvin, and V is the shared volume. Equivalently, P1 = (mole fraction of gas 1) x P_total.
It depends on the number of moles of that gas and on the shared V and T. It does not depend on the molecular mass or the identity of the gas.
No. It applies only to non-reacting (chemically inert) gas mixtures. If gases react, the number of moles changes, so simple addition of original partial pressures does not hold.
Partial pressure of a gas = its mole fraction times the total pressure. For gas 1, P1 = (n1 / n_total) x P_total. The mole fractions of all gases add up to 1.
