You can use Vander waals equation

(P+an2/V2)(V−nb)=nRT

(P+an2/V2)(V-nb)=nRT

N=number of moles

R=.082, T=273+25=298

Suppose 4.5 gram of Ammonia, Then N=4.5/17=0.264

A,b =Constant, a=4.0 atm L2mol-2, b=.036Lmol-1

you will get answer.

I would use a pressure gage. !!!!!!! This would be a sure thing because you could have leakages!

Dear Nicolaos Kokavesis,

Thank you for your response. I want to calculate theoretically.

Dear Prem Baboo,

Thank you for your response. Generally V is volume of gas, But if I am using in a reactor/ cylinder, then how to calculate V?

By using ASPEN plus we calculated the pressure in a tank. Working good, but theoretical calculation is some difficult. Because NH3 properties varies with density, ratio of vapor to liquid, temperature and pressure. For example, to check the vapor/liquid density, first we should now the exerted pressure at that temperature.

The (equilibrium) partial pressures of H2O and NH3 over ammonia solutions can be obtainined after Tables 2.21 and 2-23 in: R.H. Perry, D.W. Green, J.O. Maloney (Eds.), "Perry's Chemical Engineers’ Handbook", 7th ed., McGraw-Hill, 1997. Interpolation may be required; preferably as ln(|Pv|) or log10(|Pv|) vs. 1/T (K-1). Molar percentages for H2O above the ammonia solution are also available; after Table 2.22 (ibidem).