# Do the units of the law of mass actions convey any meaning?

Since the number that you end up with, when calculating the value of the law of mass action for a given reaction often also has a unit, I was wondering if there is any physical meaning to this given unit or if it is basically "useless"?

Example:

$$\ce{CH3COOH <=> CH3COO- + H+}$$

$$K_c = \frac{c(\ce{CH3COO-})\cdot c(\ce{H+})}{c(\ce{CH3COOH})}\qquad [K_c] = \pu{mol L-1}$$

Well, you can calculate difference in number of moles of gases and active liquid species on reactant and product side species using units of $$K_c$$ and $$K_p$$, respectively, as these are mostly used for homogeneous reactions.

For example, the unit of $$K_c$$ is $$\text{(concentration)}^n$$ if $$n > 0$$, more liquid or aqueous (ionic) species on product side.

Then, you can predict whether the product is formed or reactant when you increase or decrease external pressure or volume of a container containing a mixture in equilibrium. Temperature's effect would be only heat of reaction dependent.

Hint: write $$K_c$$ in terms of moles and volume to check effect of volume, write $$K_p$$ in terms of mole fractions and total pressure to check effect of external pressure.