A SAMPLE CONTAINS 200 ATOMS OF HYDROGEN, O.O5 GRAM ATOM OF NITROGEN ,10 TO THE POWER -20 GRAM OF OXYGEN.WHAT IS THE APPROXIMATE NO.OF TOTAL ATOMS ?

Number of hydrogen atoms = 200

Gram atom of nitrogen = 0.05g
14 grams atom of nitrogen = 6.022$\times$10²³ atoms of nitrogen
Now 0.05 grams atom of nitrogen will have = $\frac{6.022\times {10}^{23}}{14}\times 0.05 = 0.0215 \times {10}^{23} \mathrm{atoms} \mathrm{of} \mathrm{nitrogen}$

Oxygen = 10^-20 g
​16 grams of oxygen = ​ 6.022$\times$10²³ atoms of oxygen 
10^-20 grams of oxygen will have = $\frac{6.022\times {10}^{23}}{16}\times {10}^{-20} = 376.38 \mathrm{atoms} \mathrm{of} \mathrm{oxygen}$

Total number of atoms = 200 + (0.0215$\times$10²³) + 376.38 = 2.15$\times$1021 atoms