Question

Which statement describes a limitation of the kinetic-molecular theory for a gas?
a. The theory assumes that particles do not experience intermolecular forces.
b. The theory states that pressure is inversely proportional to volume.
c. The theory assumes that particles are in random and continuous motion.
d. The theory states that pressure increases with temperature.

Answer 1

we need to start by understanding what the theory actually is saying.

1. gases contain small particles that travel in straight line motion.

2. the molecules of gases occupy zero volume

3. the collisions between gases are elastic. this means that no energy is lost due to the collisions.

4. the gases don't attract or repel one another.

5. the average kinetic energy is 3kT/2 k = boltzmann constant.

Answer 2

@kmaki20

I hope you take the time to read this.

answer choices b and d are based off of the formula pV = nRT which has its roots in the molecular theory for gases.

let's look at the ideal gas equation:

p = pressure atm,
v = volume liters,
n = number of moles,
R = gas constant 0.08,
T = temperature in kelvin.

here it is below. We can study any two variables, say pressure and temperature, as long as we keep the third variable constant.

pV = nRT


so if we want to say study temperature and pressure right? we must keep volume constant.

we do some re-arranging,

pV = nRT

becomes, and VnR is just a constant. What this formula tells us is that at constant volume temperature and pressure, are directly related. if we increase the pressure the temperature will go up at constant volume.

\[\frac{ P_{1} }{ T_{1} } = \frac{ P_{2} }{ T_{2} }\]

say if we wanted to know the relationship between pressure and volume at constant temperature.

the formula becomes

p1v1 = p2V2

this means that if we increase the pressure, the volume has to go down. and if we increase the volume the pressure goes down.

Take a look at the figure below. you can see pressure and volume are inversely related below.



then we look at choice C. gases are always in a constant motion, they move around because they are generally free of intermolecular forces. so that is correct.


Choice A however, gases do not experience intermolecular forces is kind of true and not true. this is where the limitations come into play. gases do have volume, and they do take up space, but they also interact with each other. Gases will hit the walls of the container and there will be attractions and repulsions between gas particles. This is real gas behavior.