[5h00p3r_man]

I'm not quite sure on the steps required to get the solution for the following questions.

The density of a gas is found to be 2.07 g/L at 30C, and 203 kPa pressure. What is its density at STP?

250 cc of a gas is collected over acetone at -10C and 113 kPa pressure. If the gas weighs 1.32 g and the vapor pressure of acetone at -10C is 5.2 kPa, what is the molecular wight of the gas?

[enahs]

1)
PV=nRT
Let D= density and m= molar mass
That implies
P=DRT/m

So now you have P1=D₁RT₁/m and P₂=D₂RT₂/m
Setting both equal to zero (and therefor each other) and solving for D₂ you get D₂ = D₁T₁P₂/P₁T₂


2)
You have P, V, R and T, just solve for n (number of moles), and then divide the mass of the gas by the moles to get g/mol.
You just have to remember that for an ideal gas the pressure of a mixture is the sum of all its components, you know the total pressure and the pressure of one of the components (this is a binary mixture) therefore it is just subtraction to get the pressure of the component you are interested in (total-other).

[Donaldson Tan]

1)
PV=nRT
Let D= density and m= molar mass
That implies
P=DRT/m

D = m(n/V)

[enahs]

1)
PV=nRT
Let D= density and m= molar mass
That implies
P=DRT/m

D = m(n/V)

Are you just trying to help him understand that step or trying to say it is wrong? As both are correct. Though I guess it could be put more clearly as P= (D/m)RT, but mathematically they are the same.

[5h00p3r_man]

1)
PV=nRT
Let D= density and m= molar mass
That implies
P=DRT/m

So now you have P1=D₁RT₁/m and P₂=D₂RT₂/m
Setting both equal to zero (and therefor each other) and solving for D₂ you get D₂ = D₁T₁P₂/P₁T₂


2)
You have P, V, R and T, just solve for n (number of moles), and then divide the mass of the gas by the moles to get g/mol.
You just have to remember that for an ideal gas the pressure of a mixture is the sum of all its components, you know the total pressure and the pressure of one of the components (this is a binary mixture) therefore it is just subtraction to get the pressure of the component you are interested in (total-other).

OK thanks.

I was wondering for question 2, so the pressure I put in my equation is 113-5.2? Or 113+5.2?

[Yggdrasil]

You would use 113-5.2.  An important property of a mixture of gases, is that the partial pressures of each gas in the mixture adds.  For example, if gas A has a partial pressure of P_a and gas B has a partial pressure of P_b, then the total pressure P of the system will be:

P = P_a + P_b

[5h00p3r_man]

OK. Thanks.

I've got another question.

The pressure of the atmosphere 100miles above the earth is about 2.7 x 10^-7 kPa, and the temperature is about -180C. How many molecules are there in 1 cc of gas at this altitude?

I put down:

2.7 x 10^-7 x .001 = n x 8.31 x 93

But it doesn't give me the right answer...

[Yggdrasil]

You need to make sure all your units match up.

[5h00p3r_man]

How do my units not match up?

[Yggdrasil]

You are calculating the answer (n) in moles.  The question asks for molecules.

[5h00p3r_man]

This is where I'm retarded.

What's the difference between a mole and a molecule. I thought they were the same thing.

[Hello]

A mole is just an amount (like pi is in maths): just means 6.02 x 10²³ particles

A molecule isnt an amount, its a 'thing' - "The smallest unit of a substance that can exist alone and retain the character of that substance"

[5h00p3r_man]

So I just times n in my equation by 6.02 x 10^23 to get the answer?

[Hello]

I dunno, Im not an expert.

If you have n, and times it by 6.02 x 10²³, then you will get the number of particles, which needless to say will be loads.

Sorry Im not much help.