Which of the following represents a Freundlich adsorption isotherm?

1. $\frac{x}{m}=k{p}^{0.3}$

2. $\frac{x}{m}=k{p}^{2.5}$

3. $\frac{x}{m}=k{p}^{-0.5}$

4. $\frac{x}{m}=k{p}^{-1}$