Gas laws help

As part of my AS Physics homework a week or so ago, I was set the question:

3. A fixed mass of gas has a volume of 144 cm3 at 15ºC. Calculate its volume at

(a) 33ºC
(b) 0ºC, and
(c) -67ºC (the pressure being constant).

Yesterday the whole class was asked to re-attempt questiosn 3 and 4 using temperatures in Kelvin instead of Celcius.

When I was trying to figure out the question the first time around I tried using K but found that the answers didn't make logical sense as I thought that as temperature increases so does the volume of gas.

Originally, I used V = kT and calculated k to be 9.6. I the got the three answers to equal 317, 0 and -643 cm^3. Obviously, I now know that this is wrong.

However, whenever I try to calculate the answer using a temp. in Kelvin (by adding 273 degrees to the celcius temp), I get k to equal 0.5 and this figure makes the volume decrease as temp. increases.

Can anyone please tell me what I'm doing wrong?

Any help will be highly appreciated.

Thank you.

Cathy

Reply 1

JayEm

14

I answered this question for you the other day, you probably missed it.

Anyways:

JayEm

Use the relationship:
V₁/T₁=V₂/T₂

*Remember the temperature has to be in degrees Kelvin.*

So for the first one,

144/(15+273) = V₂/(33+273)

Rearrange to find V₂
V₂ = 153 cm³

Etcetera, etcetera

Make sense? :smile:

Reply 2

CathyLou

OP

2

Thanks so much. I get it now. :smile:

Yeah, I missed it the other day. Sorry that you had to repeat yourself.

Cathy

Reply 3

CathyLou

OP

2

I know that this is not on the same topic, but we were also asked to do:

7. An electric heater immersed in some water raises the temperature from 40ºC to 100ºC in 6 min. After another 25 min it is noticed that half the water has boiled away. What value does this experiment give for the specific latent heat of vaporisation of water?

I had no idea at all how to answer it so all that I wrote down was a few equations.

Change in energy = mass x the latent heat of vaporisation

Change in energy = mass x specific heat capacity x change in temperature

Mass = density x volume

Could you possibly give me some hints?

Thanks.

Cathy

Reply 4

F1 fanatic

16

CathyLou

I know that this is not on the same topic, but we were also asked to do:

7. An electric heater immersed in some water raises the temperature from 40ºC to 100ºC in 6 min. After another 25 min it is noticed that half the water has boiled away. What value does this experiment give for the specific latent heat of vaporisation of water?

I had no idea at all how to answer it so all that I wrote down was a few equations.

Change in energy = mass x the latent heat of vaporisation

Change in energy = mass x specific heat capacity x change in temperature

Mass = density x volume

Could you possibly give me some hints?

Thanks.

Cathy

Well you know the specific heat capacity of water, 4.2KJ/mol if I recall correctly, and it speaks only in terms of proportions so if you designate a mass of water M say, then it should cancel out later. Now from your second equation you can get the power output of the heater (changed slightly to P=m*C*T/t now time is involved) then use your first equation with again the power of the heater (and so the total energy) now known. M will cancel out and so you can get the latent heat (remember it only acts on half the mass now - the bit that is evaporated).

make sense?

Reply 5

CathyLou

OP

2

Thanks very much. I get it now! (Yay!)

Cathy

:tsr2:

Gas laws help

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