Q1. Calculate the number of atoms in

a) 0.5 mole atoms of nitrogen

b) 0.2 mole moleules of nitrogen

c) 3.2g of sulphur

d)3.4 g of H2S

Q2. how many molecules of water of hydration are present in 252 mg of oxalic acid?

1) (a) 1 mole of nitrogen atoms = 6.022 $\times$10²³ atoms
0.5 moles of Nitrogen atoms = 6.022 $\times$10²³ $\times$ 0.5  = 3.011$\times$10²³ atoms

(b) 1 mole of nitrogen molecules  = ​ 6.022 $\times$10²³  molecules
0.2 moles of nitrogen molecules = 6.022 $\times$10²³ $\times$ 0.2  = 1.2044$\times$10²³ molecules
As 1 molecule of nitrogen contains 2 atoms of nitrogen.
So, ​1.2044$\times$10²³ molecules of nitrogen will contain 1.2044$\times$10²³$\times$ 2 atoms
= 2.4088 $\times$10²³ atoms

(c) 32 g of sulphur (Molar mass of Sulphur ) = 6.022 $\times$10²³ atoms
3.2 g of sulphur  =$\frac{3.2 \times 6.022\times {10}^{23}}{32}$
= 0.6022 $\times$10²³ atoms

(d) 3.4 g of H2S
Molar mass of H2S = 34 g mol^-1
34 g of H2S contains 6.022$\times$10²³ molecules
1 molecule of H2S contain 2 atom of hydrogen and 1 atom of sulphur.
34 g of H₂S contain 2$\times$6.022$\times$10²³ atoms of hydrogen and 6.022$\times$10²³ atoms of Sulphur.
3.4 g of H₂S contain 12.044$\times$10²² atoms of hydrogen and 6.022$\times$10²³ atoms of sulphur.
2) Molecular formula of oxalic acid is H₂C₂O₄.2H₂O
and molar mass of oxalic acid  = $[2\times 1 +12\times 2+4\times 16 + 2\times 18] = 126$ gmol^-1
water molecule in 1 mole oxalic acid  = 2 molecules
So, 126 g of oxalic acid contains 2 $\times$ 6.022 $\times$10²³ molecules of water. 
thus, 0.252 g of oxalic acid will contain = $\frac{0.252 \times 2\times 6.022\times {10}^{23}}{126}$
=  2.409  $\times$10²¹ molecules of water