# Destroyer Gen chem #59

#### alanan84

##### D1
10+ Year Member
5+ Year Member
Consider a vessel with argon gas. This vessel is now replaced with helium gas. If both vessels contain 10 moles of gas, which is true?

A) The pressure of the container will change
B) The temperature of the container will change
C) The density of the container will change
D) The number of molecules in the container will change
E) More than one of these

I chose E b/c I knew the density would change and I figured that lighter molecules would produce weaker collisions with the container and a lower pressure. Can someone tell me what's wrong with that assumption?

#### Shinpe

Consider a vessel with argon gas. This vessel is now replaced with helium gas. If both vessels contain 10 moles of gas, which is true?

A) The pressure of the container will change
B) The temperature of the container will change
C) The density of the container will change
D) The number of molecules in the container will change
E) More than one of these

I chose E b/c I knew the density would change and I figured that lighter molecules would produce weaker collisions with the container and a lower pressure. Can someone tell me what's wrong with that assumption?
Well if at the same temperature, they're both gonna have the same kinetic energy right? And so the lighter molecules are gonna be going faster and result in more collisions. Since they both have the volume, and temperature, the pressure will be the same, but yes density will change and the molecules average velocity will change also, just not their average kinetic energy.

#### nze82

Consider a vessel with argon gas. This vessel is now replaced with helium gas. If both vessels contain 10 moles of gas, which is true?

A) The pressure of the container will change(F)...PV = nRT..Pressure is directly proportional to number of moles, and since that doesn't change the pressure won't change.
B) The temperature of the container will change(F)...Temperature is not changing.
C) The density of the container will change(T)...you knew this.
D) The number of molecules in the container will change(F)...number of moles is a direct representation of the number of molecules. Since number of moles doesn't change, the number of molecules remains the same.
E) More than one of these

I chose E b/c I knew the density would change and I figured that lighter molecules would produce weaker collisions with the container and a lower pressure. Can someone tell me what's wrong with that assumption?
Pressure doesn't really have much to do with the intensity of the collisions, it has more to do with the frequency of collisions, which is why pressure is directly proportional to the number of molecules in terms of moles.

#### Gunacaik

2+ Year Member
Pressure doesn't really have much to do with the intensity of the collisions, it has more to do with the frequency of collisions, which is why pressure is directly proportional to the number of molecules in terms of moles.
bump.

In PV = nRT, yes the pressure, temp, volume, and number of molecules won't change if we have the # of moles don't change. But if we were to use the P(MW) = DRT equation, this does show that changes in the Molecular Weight is proportional to the density.... but wouldn't it be inversely proportional with pressure? So a lower molecular mass would lead to a higher pressure? This is the part where I'm confused. Or do we only use P(MW) = DRT when solving density-related problems only?

#### Gunacaik

2+ Year Member
I just realized P(MW) = DRT is a form of the PV = nRT equation ha..... nevermind.

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