# Expression and Units Of Equilibrium Constant

Expression for Equilibrium constant ( K )

The active mass of a pure solid is constant irrespective of its amount and if a pure liquid is present in excess, its active mass is also constant.In both the cases we put active mass equal to 1.

Molar concentration of a substance means mol L-1  of the substance which is obtained by dividing the amount of the substance in moles by the volume of the substance in litres.

Molar Conc. = Moles of the substance / Volume of the substance

Molar Conc. =(  Mass of the substance / Molecular mass ) / Volume of the substance

Molar Conc. = Density of the substance/ Molecular mass of the substance

As density of a particular pure substance at a particular temperature is constant and molecular mass of the substance is also constant, therefore, molar concentration is constant.

For Homogeneous Equilibrium

CH3COOH ( l ) + C2H5OH ( l )   CH3COOC2H5 ( l ) + H2O ( l )

NH3 ( g ) + 5O2 ( g )  4NO ( g ) + 6H2O ( g )

For Heterogeneous Equilibrium

Units Of Equilibrium Constant

Keq = ( mol L-1 ) c+d / ( mol L-1 ) a+b

Keq =( mol L-1 ) (c+d ) – (a + b )

Keq = (mol L-1 ) Δn

Kp = (atm) (c+d)  / (atm)a+b

Kp = (bar) (c+d)  / (bar)a+b

Kp = ( atm or bar )(c+d) – (a+b)

Kp = (atm)Δn  or (bar)Δn

If Δn =0 i.e. number of moles of products = number of moles of reactants , Keq or Kp will have no units.

For Ex:

1) H2 ( g ) + I2 ( g )  2HI ( g )

Δn =0 , Kc or Kp will have no units.

2) N2 ( g ) + 3H2 ( g )  2NH3 ( g )

Δn = 2 – ( 1+ 3 ) = -2

The units will be ( mol L-1 )-2 or atm-2 or bar-2