Question

Which of the following observations indicates that attractive forces may exist between gas molecules?

 X. Doubling the pressure on H2(g) results in halving of its volume.  

Y. The measured molar volume of Cl2(g) is < 22.4 L at STP.  

Z. When NH3(g) is placed under 10.0 atm pressure at 25 °C, the gas condenses into a liquid.

Answer 1

In order to show that attractive forces exist between gas molecules we need to see which options does not follow ideal gas behaviour because in case of ideal gas model we assume there is no intermolecular force of attraction between molecules.

SO lets check options.

X. It shows Boyle's law relation of PV=constant which is ideal gas behaviour as doubling pressure make volume half.So it cannot indicate attractive force.Hence it is incorrect.

Y.At STP the measured volume of Cl₂(g) is < 22.4 litres this shows attractive forces because at partial pressure of atm the Cl2 molecules will have less kinetic energy to move to the walls and hence will occupy less volume.So this option is correct.

Z.Due to high pressure ,attractive forces will cause ammonia to condense due to shorter distance between molecules.Hence this option is also correct.

HENCE Y AND Z REPRESENTS ATTRACTIVE FORCES ONLY.