Answer:
-12.5 kJ/mol
Step-by-step explanation:
The free-energy predicts if a reaction is spontaneous or not. If it is, ΔG < 0. When a reaction happens by steps, the free-energy of the global reaction can be calculated by the sum of the free-energy of the steps (Hess law). If it's needed to operations at the reaction the same operation must be done in the value of ΔG (if the reaction is inverted, the signal of ΔG must be inverted).
Phosphocreatine → creatine + Pi ∆G'° = –43.0 kJ/mol
ATP → ADP + Pi ∆G'° = –30.5 kJ/mol (x-1)
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Phosphocreatine → creatine + Pi ∆G'° = –43.0 kJ/mol
Pi + ADP → ATP ∆G'° = 30.5 kJ/mol
The bold compounds are in opposite sides, so they'll be canceled in the sum of the reactions:
Phosphocreatine + ADP → creatine + ATP
∆G'° = -43.0 + 30.5
∆G'° = -12.5 kJ/mol