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.