Conversion of nitre into nitric oxide involves the following steps:
$$[KNO_3+H_2SO_4 \to KHSO_4+HNO_3] \times 2$$
$$2HNO_3 \to H_2O+2NO+3[O]$$
$$[2FeSO_4+H_2SO_4+[O]\to Fe_2(SO_4)_3+H_2O]\times 3$$
____________________________________________
$$2KNO_3+6FeSO_4+5H_2SO_4 \to 2KHSO_4+3Fe_2(SO_4)_3+4H_2O+2NO$$