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Sugafoot79

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I'm having trouble determining the molecular weight of an unknown gas using density. Ex:

What is the molecular weight of an unknown gas if 2.5g of it occupies 2L at 630 torr and a temperature of 600K?

I thought the equation P(MW)=DRT would be enough, but obviously I'm missing something when attacking these types of problems:confused:. The answer is:

74.1g/mol
 

joonkimdds

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I got 74.2 using ur formula. I think that you made mistake by not converting them to the right units.
 

Sugafoot79

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haha!!! Thats what's pent up inside of me studying for the test. lol!

another question... this is throwing me off as well.

The following rxn represents the production of hydrogen chlorine gas.

H2 + Cl2 --> 2HCL

How many grams of chlorine gas are needed to produce 3L of HCL gas at a pressure of 2 atm and a temperature of 19*C?


What's throwing me off is looking at the rxn involved. I keep coming up with the wrong answer because i'm considering 2 moles of chlorine. Is that why i'm wrong?...

The answer is:

8.88g
 
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UndergradGuy7

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I think the answer is 8.76g

That is right I think. The problem says "chlorine gas." Chlorine gas is Cl2. So in your problem there is a coefficient of 1 for Cl2.

do n= (PV) / (RT)
so n= (2*3) / (0.08*292K) = 0.25 moles of HCl

Now you can take into account the coefficients in the equation and use dimensional analysis.

0.25 moles HCl x (1 mol Cl2 / 2 moles HCl) x ( 70 g Cl2 (MW) / 1 mol Cl2 ) = 8.75 grams of Cl2
 

Sugafoot79

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How about when using the equation P(mw)=DRT...
can you set it up like this?

P(mw)=(g/l)RT --> g=(P(mw)L)/(RT)

For mw of chlorine (Cl2) I use 35g...correct? and not 70g...right?

or should I set it up like you, UndergradGuy7?
 

Sugafoot79

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Thanks for your help ya'll!!!

More on gases... A few questions:

1) A sample of helium is in a container with a movable piston. If the temperature is increased from 5*C to 10*C while keeping pressure constant, the volume of the gas will then be?

PV=nRT (Ideal Gases)

...so it goes from 278K --> 283K.

The answers says: greater than one-half, but less than twice the original volume.

Can anyone care to explain how this is greater than one half, but less than twice the volume. I might need to see it from someone's perspective.

2)A mixture of two moles of oxygen gas, 5 moles of neon, 3 moles of hydrogen gas, 2moles of nitrogen gas, and 5 moles of helium gas is in a rigid container. If the container has a pinhole, then the concentration of which gas will increase most with time?

Ans: Oxygen

I know the effusion rate will be the slowest for the heavier gas in the mixture, but isn't Neon heavier than Oxygen? Thus, it should be the most in the container over time... right?

3)At low temperature and high pressures, the measured pressure of a gas is:

Ans: less than the calculated pressure, because intermolecular forces reduce the average force of the collisions with a unit area of the container.

Are there any exceptions to the kinetic molecular theory I'm missing? I totally don't understand the reasoning behind this. Can someone please elaborate?
 

nze82

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Thanks for your help ya'll!!!

More on gases... A few questions:

1) A sample of helium is in a container with a movable piston. If the temperature is increased from 5*C to 10*C while keeping pressure constant, the volume of the gas will then be?

PV=nRT (Ideal Gases)

...so it goes from 278K --> 283K.

The answers says: greater than one-half, but less than twice the original volume.

Can anyone care to explain how this is greater than one half, but less than twice the volume. I might need to see it from someone's perspective.

V1/T1 = V2/T2

V2 = V1(T2/T1) = V1(283/278) = 1.02V1 (greater than 0.5x but less than 2x)

2)A mixture of two moles of oxygen gas, 5 moles of neon, 3 moles of hydrogen gas, 2moles of nitrogen gas, and 5 moles of helium gas is in a rigid container. If the container has a pinhole, then the concentration of which gas will increase most with time?

Ans: Oxygen

I know the effusion rate will be the slowest for the heavier gas in the mixture, but isn't Neon heavier than Oxygen? Thus, it should be the most in the container over time... right?
Oxygen gas exists as O2 (16x2 = 32 g/mole), while Ne being an inert gas exists as Ne (20x1 = 20). So oxygen gas is heavier-->Moves slower-->Efusion rate lower-->Higher concentration over time.

3)At low temperature and high pressures, the measured pressure of a gas is:
The ideal gas law works under high temp and low pressure. Under low temp and high pressure, the gas molecules come close to one another and the Van der Waal forces become significant. In other words, gas molecules are pulled together. Subsequently, the real gas volume will be lower than what's expected by the ideal gas law.
If the pressure becomes extremely high, then the gas molecules are pushed together even further. Under such circumstances, the volume of individual gas molecules becomes significant (Remember that the ideal gas law assumes that the volume of individual gas molecules is insignificant relative to the distance between them). Under such circumstance we can no longer ignore the volume of gas molecules, and this additional volume contributes to the total volume of the gas-->Larger volume relative to what's expected by the ideal gas law.

Ans: less than the calculated pressure, because intermolecular forces reduce the average force of the collisions with a unit area of the container.

Are there any exceptions to the kinetic molecular theory I'm missing? I totally don't understand the reasoning behind this. Can someone please elaborate?
Hope this helps!
 

oviedolion

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for the first one if you work it out you'll get 1 which is higher than one half but less than 2. for the second one you gotta find mass per mole which ends up being higher for oxygen i believe and the last question is definition for real gases.
 

Sugafoot79

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Sweet! Thanks, nz! I got 1 and 3 down, but not so sure about the 2nd question.

In the second question, do we need to consider the amount of moles in the container? Because if we did, 2 moles of O2 would give us 64g while 5 moles of neon will give us roughly 100g. So according to what your saying is, disregard the number of moles in the container and only consider the molecular weight of the atom...right?
 

nze82

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Sweet! Thanks, nz! I got 1 and 3 down, but not so sure about the 2nd question.

In the second question, do we need to consider the amount of moles in the container? Because if we did, 2 moles of O2 would give us 64g while 5 moles of neon will give us roughly 100g. So according to what your saying is, disregard the number of moles in the container and only consider the molecular weight of the atom...right?
I don't think you need to consider the # of moles, because the equation that relates the efusion rate to molecular weight doesn't involve anything about # of moles.
Simply considering the fact that O2 is comprised of two Oxygen atoms; whereas, Ne simply has one atom should give you the proper molecular weights to come up with the correct judgment.
 

Sugafoot79

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I don't think you need to consider the # of moles, because the equation that relates the efusion rate to molecular weight doesn't involve anything about # of moles.
Simply considering the fact that O2 is comprised of two Oxygen atoms; whereas, Ne simply has one atom should give you the proper molecular weights to come up with the correct judgment.

I get it! so effusion rate only considers MW and not its mass, right? That really threw me off.
 
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