When 10.00g of iron is burned in O2(g) to form Fe3O4(s), enough heat is generated to raise the temperature of 851g of water from 18.0 ∘C to 38.0 ∘C. What is the heat of formation of Fe3O4(s)?

##DeltaH_f=-1200"kJ/mol"##

##3Fe+2O_2rarrFe_3O_4##

The heat produced:

##=mctheta=851xx4.2xx(38-18)=71484"J"##

From the equation 3mol ##Fe## produces 1mol ##Fe_3O_4##

Converting moles to grams:

##(3xx56)grarr[(3xx56)+(4xx16)]g##

##168grarr232g##

We know that:

##10grarr71484"J"##

So to form 1 mole ##Fe_3O_4##:

##168grarr(71484xx168)/(10)=1200000"J"=1200"kJ"##

Since heat is produced:

##DeltaH_f=-1200"kJ/mol"##