# Homework Help: Total heat capacity of an monatomic gas?

1. Jul 29, 2010

### coffeem

when working this out, is it just Cv + Cp? giving 4nR???

thanks

2. Jul 29, 2010

### Gokul43201

Staff Emeritus
Please use the provided template. We can not help unless we know the exact question that was given to you - sometimes, even a minor rewording of the question can confuse matters.

3. Jul 29, 2010

### coffeem

Sorry - I wasnt really trying to find the answer to the question. It was more of a: if it says the total heat capacity, does it mean Cv + Cp or just Cp (because this is bigger).

However the question was:

a light bulb at 20 degrees is filled with a monatomic gas, as a pressure of 76e3 Pa. When the bulb is switched on the temp changed to 200 degrees.

a) what is the pressure of the gas at this temp.

I realised that i had to use pv = nrt, since the volume is fixed the pressure must also go up by a s.f. of 10.

b) if the volume of the bulb is 125cm^3, estimate the total heat capacity.

So I rearanged the ideal gas law and found there to be 2.4 moles. All I was unsure about was at the point, do I work out Cv = 3/2nR + Cp = 5/2nR and sum them???

4. Jul 29, 2010

### vela

Staff Emeritus
This isn't correct unless the temperatures were given in units of K or R, which is unlikely given the numbers involved.
No, the question is asking for how much heat the bulb must absorb to raise its temperature by 1 degree.

By the way, don't write "Cv = 3/2nR + Cp = 5/2nR" unless you really mean that Cv=5/2nR and Cp=R because that's what it means. I know a lot of students tend to do this, but it's really sloppy notation and often leads to mistakes.

5. Jul 29, 2010

### coffeem

Your corrct it was in degrees celcius.

Umm... how would I go about working that out then? thanks

6. Jul 29, 2010

### vela

Staff Emeritus
Try taking a stab at it and posting your attempt here. You have the right idea for part a; you just have to use the absolute temperatures, i.e. convert them to K.

7. Jul 30, 2010

### coffeem

OK.

So I have converte the temps to K:

T1 = 293K
T2 = 473K

So T2/T1 = 1.61

So by my reasoning the pressure will now be: 12.3ee4Pa

However how do I go about doing the second part? thanks

8. Jul 30, 2010

### coffeem

Ok doing this more properly now. I get the pressure to be: 12.3ee4Pa

Which gives me 0.39moles.

So working out: Cv = 3/2nR = 4.86???? is this correct?

9. Jul 30, 2010

### vela

Staff Emeritus
No, because you didn't specify any units.