equation solving – FindFit;;nrlnum problem


I am trying to fit my experiment data to the equation but I have a problem as below. Could somebody can help me with this? I would really appreciate it!

R = 8.3145;
dH1 = -143683;
dS1 = -186.036;
dG1(T_) = dH1 - (T + 273) dS1;
keq1(T_) = E^(-dG1(T)/(R (T + 273)));
S = 0.2;

This is my data that I want to fit.

solveair = {{650., 0.0929321}, {625., 0.0916715}, {600., 0.090182}, {575., 
  0.0882618}, {550., 0.0850123}, {525., 0.0823273}, {500., 
  0.0781639}, {475., 0.0694796}, {450., 0.0457255}, {425., 
  0.0328606}, {400., 0.0221124}}

And this is the fitting code I used and had a problem.

airfit = FindFit(solveair, 
  1/24 (E^((-dH1 + dS1 (T + 273))/(R (T + 273))) Sqrt(po2) + 12 S - 
     Sqrt((E^((-dH1 + dS1 (T + 273))/(R (T + 273))))^2 po2 + 
      24 E^((-dH1 + dS1 (T + 273))/(R (T + 273))) Sqrt(po2)
        S)), {po2}, {T})

Thanks in advance!