$\therefore 12\text{ moles of }\mathrm{H}\text{-atoms }=1 \mathrm{mole}\text{ of }{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$

$\therefore 1\text{ mole of }\mathrm{H}\text{-atom }=\frac{1}{12}\text{ mole of }{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$

$\therefore 6.36\text{ moles of }\mathrm{H}\text{-atom }=\frac{1}{12}\times 6.36$

                                          $=\frac{6.36}{12}\text{ mole of }{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$

1$\text{ mole of }{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4=4\text{ moles of oxygen }$

$\text{ So, }\frac{6.36}{12}\text{ mole of }{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4=\frac{4\times 6.36}{12}=2.12 \mathrm{mol}$