# Universal gas constant for 1 kg oxygen

1. May 8, 2013

### Ezio3.1415

We know, the unit for R=... J / mol K

But in a math problem, they asked for universal gas constant for 1 kg of oxygen... They gave pressure,density and temperature...

They got the unit of R J/kg K as they used
p=(density) R T
What does R mean here?

But I used
pv=nRT
p=(density)*R*T/M(atomic mass)
and found the R to be ... J/mol K

I can change the unit and the answer becomes same then... but what is the right answer given the question...

2. May 8, 2013

### UVW

This doesn't sound like a very good problem. It seems that they have simply redefined the gas constant in terms of oxygen, but I've never known this to be a very useful thing to do at all. If that's a convention that you need to know for your courses, it'd be wise to learn, but I wouldn't stress about understanding it.

It sounds like you understand where they got their answer, but correct me if I'm wrong.

3. May 8, 2013

### Staff: Mentor

Such redefined gas constants are used in some branches of science and engineering. I remember having troubles reading a textbook on atmosphere physics for this reason. I agree it doesn't make much sense, as it hides the universality of the R.

4. May 8, 2013

### Ezio3.1415

correction: I meant molecular mass by 'M'... But wrote atomic mass... >_<

I read about it and got the answer... Actually they found out the specific gas constant which is R divided by M(molecular mass)... R(specific) is different for different gases... As the ques asks for gas constant for 1 kg I think they meant this... But it led to confusion as they added 'universal'... Rspecific is not universal...

However, I think Rspecific is the way to do it... The given data(solving the second part) fits in... on the second part of the problem they gave Cp and asked for Cv
But Cp is not given as molar specific heat(constant pressure) there... Just specific heat(constant pressure)... I mean the unit is not J / mol K ... Its J / kg K

Cp-Cv=R we all know that... Its not only true for molar specific heat also true when C means only specific heat but then the constant on the right side is R specific...
I mean for molar C, Cp-Cv=R
For normal C, Cp- Cv=R specific

I think that is it... What do u think?