text: First Balance the formula and make a formula:\n\nF2(g) + 2Na(s)------------------- 2NaF(s)\n\nAtomic Mass Na(s): 22.99g/mol\n\n0.56gNa * 1mol/22.99gNa= 0.0244 mol Na\n\nSince 13.8 kJ evolved from 0.0244mol Na divide them together to obtain Kj/mol\n\ntherefore 13.8Kj/0.0244mol= 566 Kj/mol Na\n\n\nNow using the chemical coefficent from the balanced equation turn it into the stadard enthalpy of NaF.\n\n566 Kj/mol Na * 2 mol NaF(s)/2mol Na(s)= 566 Kj/mol NaF(s)\n\nSince they used the term evolved in the question it means it is exothermic and threfore the stadard heat of formation of NaF(s) is negative ( -566 Kj/mol NaF(s))