The equilibrium vacancy concentration in aluminum at $900$ $\mathrm{K}$ is $1.1 \times 10^{-4}$. The enthalpy of formation of vacancies in $\mathrm{kJ}. \mathrm{mol}^{-1}$ is $\_\_\_\_$ (rounded off to one decimal place).
Given: Universal gas constant $=8.314 \mathrm{~J} \cdot \mathrm{~K}^{-1} \cdot \mathrm{~mol}^{-1}$
