20. When 0.560 g of Na(s) reacts with excess F₂(g) to form NaF(s), 13.8 kJ of heat is evolved at standard-state conditions. What is the standard enthalpy of formation (△H°_f) of NaF(s)?
(A) 24.8 kJ/mol
(B) 570 kJ/mol
(C) –24.8 kJ/mol
(D) –570 kJ/mol

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