g-chem question about gases

[tank you]

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Which of the following affects the average velocity of gas particles?

I. Temperature
II. Pressure
III. Molecular Weight


From the formula KE(avg)=1.5RT and using kinetic molecular theory for an ideal gas, I can guess that temp affects, but what about pressure and MW?

 

[fpr85]

Which of the following affects the average velocity of gas particles?

I. Temperature
II. Pressure
III. Molecular Weight

I'm no expert on the matter but wouldn't this have a lot to do with Boyle's and Charles' Law? Perhaps more along the lines of Boyle's though...

 

[liverotcod]

Average velocity of a gas molecule is directly proportional to the temperature, and inversely proportional to its mass.
http://www.ucdsb.on.ca/tiss/stretton/chem1/gases9.html

 

[fpr85]

Average velocity of a gas molecule is directly proportional to the temperature, and inversely proportional to its mass.
http://www.ucdsb.on.ca/tiss/stretton/chem1/gases9.html

lol, you probably embarrassed him. Avert your eyes from the "Grade 11 Chemistry" 😀

 

[liverotcod]

lol, you probably embarrassed him. Avert your eyes from the "Grade 11 Chemistry" 😀

Heck, I'm still trying to master the stuff I was supposed to have learned in kindergarten!
Plus, it's Canadian grade 11, which if you believe all the Canadians around here is like US grad school.

 

[shaq786]

So what is the answer???
I want to say all 3 becuase the molecular weight...the greator the molecular weight the less it will diffuse across a room.

For pressure....the greater the pressure causes particles to be crunched together, thus causing more collsions and increasing kinetic energy.

Temp...Increase in temperature, causes an increase in speed of the particle.

But I bet you Im wrong on one of these lol. So whats the answer?

 

[liverotcod]

I and III only. Pressure is an effect caused by collisions of the molecules with the container, and has no effect on the average velocity.

 

[tank you]

thanks a lot!

 

[fpr85]

I and III only. Pressure is an effect caused by collisions of the molecules with the container, and has no effect on the average velocity.

But wouldn't decreasing the volume thus increasing the pressure cause the molecules to move faster? Or is it because the question asks for "average" velocity?

BTW, first Bio test = 94%, yay! The class average was a 65%.

 

[VPDcurt]

Heck, I'm still trying to master the stuff I was supposed to have learned in kindergarten!
Plus, it's Canadian grade 11, which if you believe all the Canadians around here is like US grad school.

Canada blows. McGill is supposedly the Harvard of Canada yet it is quite easy to get into. Academics are much tougher in the US than in Canada.

 

[fpr85]

Canada blows. McGill is supposedly the Harvard of Canada yet it is quite easy to get into. Academics are much tougher in the US than in Canada.

Aha! No wonder blake is enjoying medical school so much!

 

[hotlikebutter]

this is a hard question to answer without set conditions. but yes all 3 can affect velocity. temp and pressure are inversely related, and temp is a measure of kinetic energy/velocity, so both affect the outcome. and size matters, 😛, the smaller the molecule the faster it travels. but dont worry about questions like this on the mcat. the mcat will mostly ask you something like this:

1) At 315 K, which molelcule has the greatest velocity
a. F
b. Cl
c. Ar
d. all have the same velocity

answer is of course, d. temp is a measure of KE/velocity, so if they are at the same temp, they are at the same velocity. but if they dont give you a set condition, then you would choose the smaller atom.

 

[fpr85]

but if they dont give you a set condition, then you would choose the smaller atom.

But wouldn't the larger ones have more momentum?

 

[liverotcod]

Temperature varies with kinetic energy, not velocity. At a given temperature (in an ideal gas, I should add), a smaller molecule moves faster while a larger molecule moves slower. They both have the same kinetic energy. Pressure would only have a non-ideal effect at extremes.

 

[Nuel]

the question is vague. But all choices are correct. Pressure would be a consequence of the average velocity of the particles enclosed due to collisions, and this can be argued for the other way round.

Also consider that pressure is exerted in both x,y and z directions, and the average velocity in one direction is taken to be the case in all 3 directions so that in elastic collisions at average velocity in a 3-dimensional frame of reference P*V = N*m*u^(1/2)/3. If you consider Avogrado's number et al, u = sqrt(3RT/M).

 

[liverotcod]

Yah it's vague. I made the assumption that each item should be considered holding everything else constant, which (I think?) validates my answer.

 

[Nuel]

Yah it's vague. I made the assumption that each item should be considered holding everything else constant, which (I think?) validates my answer.

A knee-jerk response would make me say I and III, but if you consider the equations associated with the average velocity, all three are correct. It's quite a tricky question.

 

[Peterock]

Ideal behavior would make the answer I + III. Pressure would not work b/c there is no intermolecular attraction. Also... I'm guessing this is a lower level chemistry question. You should think of 2-3 seperate gas molecules as ball bearings that are totally elastic in behavior. While they may hit each other and the wall more often w/ increasing pressure, they do not gain or lose energy (E=1/2mv2).

Nonideal behavior would make the answer I, II, + III (b/c increasing the pressure would eventually cause the gas to turn into a liquid, but in general they'd slow down due to intramolecular attraction and inelastic collisions).

 

[shaq786]

So heres the rock bottom question...should we always assume ideal behavior in gases for standalone questions like these unless otherwise stated?????????????????????????????????????????????????????????????????????????????????????????????????

 

[liverotcod]

So heres the rock bottom question...should we always assume ideal behavior in gases for standalone questions like these unless otherwise stated?????????????????????????????????????????????????????????????????????????????????????????????????

You will not see a question as vague as this on the MCAT. Test-prep questions are much more poorly written than real MCAT questions. In general, though, don't overthink the exam - it's written at an intro class level - no pchem or advanced physics.

Which suggests that in this case, assuming ideal gas behavior is reasonable.