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how to calculate the heat evolved (in kJ) per gram of ZnS roasted

The first step in the industrial recovery of zinc from the zinc sulfide ore is roasting, that is, the conversion of ZnS to ZnO by heating.

2 ZnS (s) + 3 O2 (g) --> 2 ZnO (s) + 2 SO2 (g)  delta H = -879 kJ/mol

Calculate the heat evolved (in kJ) per gram of ZnS roasted.

The information you have written, tells you that for each mole of those reactions you get 879 kJ of energy. For each mole of the reaction you are using up 2 moles of ZnS. Convert the 2 moles into mass and you can easily calculate the amount of energy pr. gram ZnS.

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