The ideal gas equation - help please!

I'm stuck on this ideal gas equation and as i have just started, i find it quite hard. Please help me on this...

calculate the molar mass of gas, which has a density of 2.615gdm³ at 298K and 101kNm-²

R= 8.31 Jkmol
gas molar volume= 22.4dm³ at STP

Any help will do! Thanks!

Reply 1

non-centrally acting

11

calculate the molar mass of gas, which has a density of 2.615gdm³

The equation is PV=nRT
P= 101 kNm - you have to convert this into pascals, but i dont know how
V= 22.4 x 10^-3 (you have to convert it to m^3)
n= ???? I'm guessing molar mass is the moles??
R= 8.31
T= 298

Rearange the equation so its n= (PV)/ (RT) and put all the values in it. I dont understand the part on molar mass and the density which has been given. (sorry- hope i'm of some help!) :smile:

Reply 2

cat123

OP

1

darth_vader05

calculate the molar mass of gas, which has a density of 2.615gdm³

The equation is PV=nRT
P= 101 kNm - you have to convert this into pascals, but i dont know how
V= 22.4 x 10^-3 (you have to convert it to m^3)
n= ???? I'm guessing molar mass is the moles??
R= 8.31
T= 298

Rearange the equation so its n= (PV)/ (RT) and put all the values in it. I dont understand the part on molar mass and the density which has been given. (sorry- hope i'm of some help!) :smile:

thanks for that but I still have no idea how to convert kNm-² to Pa. Do you have to covert it to atmospheres and then to Pa? But how do you do that? Also how to you convert gdm³ to m³ and getting rid of the g? I'm really confused!

Reply 3

non-centrally acting

11

I'm sorry but thats were I got stuck aswell. I had a question like that today but waited for my teacher to convert everything for me :redface:

Hopefully someone else will be able to help you!

Just a thought although not needed for this question...
PV = nRT
P can be measured in atmospheres in which case you just use the appropriate value for R (0.062)
P can be measured in KPa (KNm^-2)

to get the correct units for R just substitute a set of values at STP
V =22.41dm3
T= 273
n=1
P = (you choose)
R = this is what the sum comes to

for example

when P is in kPa
P = 101 kPa

101.3 x 22.4 = 1 x R x 273
therefore R = 8.314

When P is in atmospheres
1 x 22.4 = 1 x R x 273
R = 0.082

any of these can be used providing that you are consistent.

---------------------------------------------------------------

So on to the problem :

density of 2.615gdm³ at 298K and 101kNm-²

we have atmospheric pressure here so that for one mole of any gas occupying 22.4dm3 at STP (V1, T1)

V1/T1 = V2/T2

V2 = V1T2/T1
V2 = 22.4 x 298/273
V2 = 24.45

so one mole of this gas would occupy 22.45 litres at 298K
as the density = 2.615gdm³
then the molar mass of the gas = 22.45 x 2.615 =63.9g (it's probably SO2)

The ideal gas equation - help please!

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