A sample of ammonium phosphate ${\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$ contains 3.18 moles of hydrogen atoms. The number of moles of oxygen atoms in the sample is

In ${\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4$, 12 moles of 'H' are present with 4 moles of oxygen atom.
$\therefore$3.18 moles of 'H' are present with $=\frac{4}{12}\times 3.18=1.06$moles of oxygen atom