The standard heat of combustion of solid boron is equal to
B(s) + 34O2(g) → 12B2O3
ΔH0r = ΔH0c = 12H0f(B2O3) − ΔH0f(B,S) − 34
ΔH0f(O2,g)
ΔH0f of elements is assumed to be zero.
ΔH0c = 12ΔH0f(B2O3)