Răspuns:
Calculam impuritatile necombustile
100g lignit...............25g impuritati
1000g lignit..............X
X=250g impuritati
Diferenta 1000-250=750g reprezinta carbonul(partea combustibila)
Calculam nr moli carbon=750:12=6,25moli
1 molCarbon..................96kcal
6,25molCarbon..............x
x=6,25×96=600kcal