ideal-gas specific heats of various common gases function of temperature

I know that the static pressure specific heat of air, an ideal gas, is 1.005, but in the one I made, it comes out to be 1.0002. What went wrong?

clc; clear all; close all;

M = 28.96;
T = 273.15;
a=28.11; b=0.1967E-2; c=0.4802E-5; d=-1.966E-9;
dcp=(a+(b.*T)+(c.*(T.^2))+(d.*(T.^3)));
cp = dcp./M;
disp(cp);

    1.0002

[Dyuman Joshi]

There's a term missing in the polynomial expansion -

[재훈]

oh thx u for your help!! I know it's difficult, but where can I find that value? I tried to find it, but I'm not sure.

Thank you verry much, and Have a good day!

[Dyuman Joshi]

Unfortunately, NIST has not published any data regarding Shomate polynomials of Air (If I had to guess why - I would say it is probably because air is a mixture of different elements and compounds, thus it woud be difficult to obtain data for it).

I could only find the values for N2, O2, H2, H2O and CH4.

[재훈]

Sorry for the late reply. Thanks for looking into the values. But I don't think it's needed now. Nonetheless, thank you for your help and have a nice day :) cheerio(I actually finished the assignment and it's all thanks to you).

Answer 1

[VBBV]

Hi @재훈 if you consider a change in sign for the last term it would result in 1.005

M = 28.9; % assune 
T = 273.15;
a=28.11; b=0.1967E-2; c=0.4802E-5; d=-1.966E-9;
dcp=(a+(b.*T)+(c.*(T.^2))-(d.*(T.^3)));
                        %changge in sign here  
cp = dcp./M;
disp(cp);

    1.0050

[재훈]

Wow thx for your help. It's really helpful. Have a good day!

[Dyuman Joshi]

Although @Angelo Yeo has accepted this answer, I do not think this is a right approach to solving this problem.