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### The value of Cp

Posted: Wed Jan 31, 2018 8:40 pm
In 9.7 I believe we have to use the equation S = nCp lnT1/T2. Are we always given the value of Cp?

### Re: The value of Cp

Posted: Wed Jan 31, 2018 8:42 pm
on the test we should be given the value of Cp and you only use Cp if it explicitly says constants pressure.

### Re: The value of Cp

Posted: Wed Jan 31, 2018 9:25 pm
Cp for ideal gases is 4R. Cv for ideal gases is 3R. When in doubt, remember that Cp = Cv + R

### Re: The value of Cp

Posted: Wed Jan 31, 2018 10:22 pm
Cp of ideal gases is 5/2(R) and Cv is 3/2(R)

### Re: The value of Cp

Posted: Wed Jan 31, 2018 10:29 pm
kaushalrao2H wrote:Cp for ideal gases is 4R. Cv for ideal gases is 3R. When in doubt, remember that Cp = Cv + R

I thought for an ideal gas Cp is (5/2)R and Cv is (3/2)R?

### Re: The value of Cp

Posted: Thu Feb 01, 2018 12:37 am
Yup--for a monatomic ideal gas, Cv = 3/2R and Cp = Cv + R = 5/2R

Note that for diatomic gases, it gets a little tricker (e.g. for N2, Cv = 5/2R) due to more rotational degrees of freedom

### Re: The value of Cp

Posted: Thu Feb 01, 2018 8:38 pm
So we only use the C value when there is a constant pressure value and use the R constant value when the pressure isnt constant?

### Re: The value of Cp

Posted: Thu Feb 01, 2018 11:06 pm
The Cp and Cv values can be found on page 281 in the textbook. Cp is the heat capacity of the system at constant pressure while Cv is the heat capacity at constant volume. R is the ideal gas constant, and you would typically use R in the form 8.3145 J*K-1*mol-1 for these calculations.

### Re: The value of Cp

Posted: Thu Feb 01, 2018 11:10 pm
On the test, you will be given the table of values that will help you solve for cP