If the reaction uses 140 grams of iron, how much rust could be produced?

4Fe(s) + 3O2(g) → 2Fe2O3(s) ΔH = -1.7 x 10^3 kJ 
shows that 1700 kJ of heat is released when 4 moles of Fe react. 

Molar mass of Fe = 56 g/mol 
Molar mass of Fe2O3 = 160 g/mol. 

(a) When 4 mol ( 4 x 56 = 224 g) iron reacted 1700 kJ are released, 
when 9.0 g Fe reacted, the amount of energy will be 
(9.0 x 1700) / (224) = 68.3 kJ 

(b) According to the equation, 1700 kJ of energy released during the formation of 2 moles of Fe2O3 which is 2 x 160 = 320 g of rust. 

120 kcal x (4.18 kJ / 1 kcal) = 501.6 kJ 

When 1700 kJ released = 320 g rust 
when 501.6 kJ released = (501.6 x 320) / 1700 = 94.4 g rust

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