Ok!

If you take a look in "Thermodynamics: An Engineering Approach", 5th edition by Yunus A. Çengel and Michael A. Boles a very interesting hanbook in thermodynamic you will find  your need and more.

In short the specifics heats Cp & Cv of a gas mixture are not that given by the sum of specifics heats of the individual gases. They are equal to the sum of (mass fraction x heat specific) related to individual gas.

Thank u for the msg.

So in order to find out the Cp value for CO2-NH3 (1:1) gaseous mixture, you mean I have to do this

Specific heat of CO2-NH3 gaseous mix= 0.5x(specific heat of CO2 + Specific heat of NH3)

If cp or cv is given in kJ/kgK  multiply by the mass fractions. If Cp or Cv is given in kJ/kmolK  multiply by the mol fractions (=volume fraction). 

Thank you all for this help.

The specific heat of mixture contains also the part, connected with interaction of molecules. At high densities and low temperatures this part may be large enough to influence the resulting heat capacity.

Yes, the simple mixing rules are only valid for the perfect gas. 

Thank you, Wilfried, for reasonable words! Now there are many dogmatic followers of simplified concepts in Thermal Physics. The Nature is more complex than the models used by many of scientists in their simulations.

The following somewhat related discussion ― on Cp/Cv ratio for reacting gases ― may possibly be found interesting by followers of this discussion:  https://www.researchgate.net/post/how_can_we_calculate_gamma_specific_heat_ratio_for_real_gas_chemical_non-equili

If it is density or viscosity or thermal conductivity of gas you have to use mole fraction instead of mass fraction

Concerning thermal conductivity of gas mixtures: Use mole fractions and logarithms of thermal conductivities.

INTEGRAL OF CP MIXTURE BETWEEN T1 T0 T2 DIVIDED BY T2 MINUS T1 WILL GIVE THE MEAN OF THE SPECIFIC HEAT CAPACITIES OF THE COMPONENTS OF THE MIXTURE , WHERE T1 IS INITIAL TEMPERATURE AND T2 THE FINAL TEMPERATURE. CP MIXTURE = A+BT+CT SQUARE + DT CUBE

WHERE T IS THE TEMPERATURE AND A,B,C,D,E..... THE SPECIFIC HEAT CAPACITY OF EACH COMPONENT OF THE MIXTURE. THE INTEGRATION OF THIS WILL GIVE YOU THE MEAN OR AVERAGE SPECIFIC HEAT CAPACITY OF THE MIXTURE. THANK YOU.

Just for the ideal gas mixture, there are a lot of academic papers working in this direction.