Question

does equilibrium constant have any SI unit​

Answer 1

Answer:

Since activities are unitless, they eliminate the units of all the quantities in the equilibrium constant expression, making the constant itself unitless all the time. ... Note that activity gives no indication of the total amount of substance present, but only of the amount present in a given volume (and temperature).

Answer 2

Recall that in the previous section we said that for any reaction:

aA + bB + ...  rR + sS + ...,

the K is defined as:

where K is a constant for the reaction at a given temperature, and that K has no units. This lack of units makes sense for any reactions with the same number of moles of products and reactants. Consider the reaction

CO2 + NO  CO + NO2

which has the equilibrium expression

Since units of all the concentrations are M, the units of this expression are M 2 ÷ M 2, which cancels to be unitless. Similarly, the reaction

H2 + I2  2HI

has the equilibrium expression

which is unitless as in the above example.

But what about most reactions, including acid-base reactions, which do not have equal moles on each side of the equation? In a typical acid-base equilibrium,

HX  H+ + X–

the equilibrium constant expression is

Here the units do not cancel– we have units of M. Different reactions give different units for K, which is very confusing. To avoid this, we introduce the concept of activity, a unitless quantity that can be used in place of concentration or pressure. Since activities are unitless, they eliminate the units of all the quantities in the equilibrium constant expression, making the constant itself unitless all the time.