The degree of dissociation (α) if a weak electrolyte, AB is related to van't Hoff factor (r) by the expression

AxBy⇌xAy++yBx- (1-α)     xα        yα  After dissociation x       i=nAxBy+nAy++nBx-         =1-α+xα+yα         =1+αx+y-1 ∴  α=i-1x+y-1