Lets write up a few things: $1 eV=1,60217653 * 10^{-19}J$ $MeV=1000000 eV =10^{6} eV$ $1 kJ=1000 J$ $N _{a}=6,02214*10^{23} \frac{ 1 }{ mol }$ So: $13.1MeV*10^{6} \frac{ eV }{ MeV }*1,60217653 * 10^{-19} \frac{ J }{ eV }*10^{-3} \frac{ kJ }{ J }*6,02214*10^{23} \frac{ 1 }{ mol }$