25 g of water contain

= 1.388 moles.

Number of H-atoms = Number of moles of $H_{2}O\times Avogadro\text{'}snumber\times 2$

$=1.388\times 2\times 6.023\times {10}^{23}$

$=16.722\times {10}^{23}$  atoms of hydrogen

Number of oxygen – atom = number of moles of $H_{2}O$  × Avogadro’s number

$=1.388\times 6.023\times {10}^{23}$

$=8.362\times {10}^{23}$  atoms of oxygen.