More practice questions

4. Q4. Find the number of molecules per cubic metre in a gas at 300 K and 1 atm (101 325 Pa).

   Show answerHide answer

   n = P/(k_BT) = 101 325/(1.38 × 10^⁻²³ × 300), about 2.45 × 10^²⁵ m^⁻³.

5. Q5. Why is it that about 22.4 L of an ideal gas at NCERT STP contains the same number of molecules, though the gases have different masses per molecule?

   Show answerHide answer

   By the ideal-gas equation, N = PV/(k_BT), which depends only on P, V and T, not on what the gas is made of. That is Avogadro’s hypothesis; the masses differ because the molecules differ in mass.

6. Q6. A vessel holds equal masses of helium (4 u) and oxygen (32 u). Find the ratio of their partial pressures.

   Show answerHide answer

   Partial pressure ∝ number of moles = mass/molar mass. So P_He/P_O₂ = 32/4 = 8.