I'm sorry, I can't make any sense of your data.
Should usually adsorbed mass equal to initial mass?
No, initial mass minus
equilibrium mass. Sorry if that wasn't clear. That is what you say you've done (x = (I) - (II) ). But your numbers don't agree with that. x is not equal to I - II.
The initial masses, if I understand your procedure correctly, ought to correspond to 5, 10, 20, 30, 40 and 50 mL AcOH, and should thus be in the ratio 1:2:4:6:8:10. But they are not; they are in the ratio 1:4:16:36:64:100. (Square of the correct ratios?)
Either your numbers are wrong, or you didn't follow the procedure you've written.
Please show, for at least one of your samples, the actual calculations
you did to get those numbers.
Otherwise, with the numbers as they stand, they seem consistent with Freundlich but not Langmuir.