The degree of dissociation (α) of a weak electrolyte, AxBy is related to van't Hoff factor (i) by the expression :
a. α = (i-1)/(x+y+1)
b. α = (i-1)/(x+y-1)
c. α = (x+y+1)/(i-1)
d. α = (x+y-1)/(i-1)