(6)

(7)

If aluminum is used to deoxidize steel,  in steel can be determined by

(8)

Combining equations 7 and 8 we get.

(9)

Consider a slag with  and  which is used to desulphurize steel at . Steel has  dissolved. Activity of  is 0.38 in slag
Let us calculate  by equation 9.

 by equation                    
 by equation

References:
A.Ghosh: Secondary Steelmaking