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Ideal gas pressure

1.

This is a question about ideal gases.

When a gas in a cylinder is compressed at constant temperature by a piston, thepressure of the gas increases. Consider the following three statements.


2.

I The rate at which the molecules collide with the piston increases.

3.

II The average speed of the molecules increases.

4.

III The molecules collide with each other more often.


Which statement(s) correctly explain the increase in pressure?


5.

A I only

6.

B II only

7.

C I and II only

8.

D I and III only

I thought the answer was D, but it is A. I don't understand this.



Original post by Aaradhana

1.

This is a question about ideal gases.

When a gas in a cylinder is compressed at constant temperature by a piston, thepressure of the gas increases. Consider the following three statements.

2.

I The rate at which the molecules collide with the piston increases.

3.

II The average speed of the molecules increases.

4.

III The molecules collide with each other more often.

Which statement(s) correctly explain the increase in pressure?


5.

A I only

6.

B II only

7.

C I and II only

8.

D I and III only

I thought the answer was D, but it is A. I don't understand this.





Well when P is increased the molecules are forced into a smaller space so they will collide with the piston more often.

II cannot be right because they haven't increased in speed or mass or temperature, so the kinetic energy remains the same.

Due to Boyle's law the increase in pressure will cause a decrease in the volume, but the number of molecules present will remain the same. So I would've thought they'd collide with each other more - because they're in a smaller space. This is confusing me too. Does it say anywhere in the question that molecules of gas escapes at all? That's the only thing I can think of.

I've searched for ages and can't work out why it's not D.
The question asks for the explanation of the increase in pressure in molecular terms.
Pressure is due to the rate of change of momentum of the molecules striking the piston.
It doesn't depend on whether or not they collide with each other more.
At the same temperature the average speed remains the same.
Reply 3
Original post by Stonebridge
The question asks for the explanation of the increase in pressure in molecular terms.
Pressure is due to the rate of change of momentum of the molecules striking the piston.
It doesn't depend on whether or not they collide with each other more.
At the same temperature the average speed remains the same.


But is it not implied that as the particles hit the walls with a higher frequency they will also hit each other with a higher frequency?
Original post by Aaradhana
But is it not implied that as the particles hit the walls with a higher frequency they will also hit each other with a higher frequency?



Yes they do, but that is not what the question asks and is irrelevant.
It asks for an explanation of the higher pressure.
The fact that they strike each other more often is a consequence of the higher pressure but is not the reason for it.

You need to see the difference between a statement that happens to be true as a result, and a statement that is the cause.

Pressure is not due to the collisions between the molecules themselves. It's due to the collisions with the container.
Reply 5
Original post by Stonebridge
Yes they do, but that is not what the question asks and is irrelevant.
It asks for an explanation of the higher pressure.
The fact that they strike each other more often is a consequence of the higher pressure but is not the reason for it.

You need to see the difference between a statement that happens to be true as a result, and a statement that is the cause.

Pressure is not due to the collisions between the molecules themselves. It's due to the collisions with the container.


Wow! I so did not think of that!!
That was soooo smart!
Thanks a lot!

PRSOM!

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