# Henry's law p = kx vs m = kp

What exactly is the difference between the following equations? $$p = kx \qquad\text{and}\qquad m = kp$$ Is the $$k$$ in both same, because I have seen same $$k$$ in textbooks for both equations?

• Both are different Henry's law constants. See: en.wikipedia.org/wiki/Henry%27s_law May 20, 2019 at 20:48
• Henry's law constants also reflect an implicit scale: molality, mole fraction, concentration, etc. (with a given reference state defined as such).
– Rolf
May 20, 2019 at 22:40

While both equations imply a relation between the partial pressure of the solute and its concentration in solution, clearly $$k$$ means different things in each.
The first, $$x=H^\mathrm{xp} p$$ is Henry's law with $$H^\mathrm{xp}=1/k$$ the Henry's law constant in units of mole fraction concentration per unit pressure. The second $$m=H^\mathrm{mp} p$$ is an alternative form with $$H^\mathrm{mp} = k$$ the Henry's law constant in units of molal concentration per unit pressure.
$$H^\mathrm{xp} = H^\mathrm{mp} M_\ce{H2O}$$