The energy released per fission of nucleus of  $200MeV$. The energy released, if all the atoms in 120g of pure  ${ }^{240}X$ undergo fission is ……………. $\times {10}^{25}MeV$. (Given, $N_A=6\times {10}^{23})$ 

Molar mass of given nucleus $\left({ }^{240}X\right)=240g$ 
So, in 240g number of nucleus present $=N_A=6\times {10}^{23}$ 
$\therefore$ In 120g, number of nucleus present $=N_A/2=3\times {10}^{23}$  
1 nucleus gives 200MeV of energy in fission. So, total energy obtained by fission of 120g of ${ }^{240}X$  will be
$3\times {10}^{23}\times 200=6\times {10}^{25}=6\times {10}^{25}MeV$