How many sulphur and oxygen atoms are present in 0.25 mole of ${\mathrm{SO}}_3$?

SO₃ has 1 sulphur atom and 3 oxygen atoms in 1 molecule.
1 mole of any substance contains molecules =$6.022 \times 10^{23}$
Number of atoms of sulphur in 0.25 mole of SO₃ = $0.25 \times 6.022 \times 10^{23}\mathit{ }$         $\mathit{ }\Rightarrow$ $1.505 \times 10^{23} \mathit{atoms}$

1.505 ×10²³ sulphur atoms are present in 0.25 mole of SO_3.

Number of atoms of oxygen in 0.25 mole of SO₃ =$3 \times 0.25 \times 6.022 \times 10^{23}\mathit{ }$                     $\mathit{ }\Rightarrow 4.515 \times 10^{23} \mathit{at}\mathit{oms}$
3 is taken because 1 molecule of SO₃ contains 3 oxygen atoms.

Total number of atoms in 0.25 moles of SO₃ $= (1.505 + 4.515) \times 10\mathit{²³ }$                                                                               $\mathit{ }\Rightarrow 6.02 \times 10\mathit{²³}$