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Calculate the number of atoms of hydrogen present in 5.6 g of urea, (NH2)2CO. Also calculate the number of atoms of N, C and O.

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Given : 

Mass of urea = 5.6 g 

To find : 

The number of atoms of hydrogen, nitrogen, carbon and oxygen 

Calculation : 

Molecular formula of urea :

(NH2)2CO

Molar mass of urea = 60 g mol-1

Number of moles = \(\frac{Mass\,of\,a\,substance}{Molar\,mass\,of\,a\,substance}\)

\(\frac{5.6\,g}{60\,g\,mol^{-1}}\)

= 0.0933 mol

∴ Moles of urea = 0.0933 mol 

Number of atoms = Number of moles × Avogadro’s constant

Now,

1 molecule of urea has total 8 atoms, out of which 4 atoms are of H, 2 atoms are of N, 1 of C and 1 of O.

∴ Number of H atoms in 5.6 g of urea = (4 × 0.0933) mol × 6.022 × 1023 atoms/mol 

= 2.247 × 1023 atoms of hydrogen

∴ Number of N atoms in 5.6 g of urea = (2 × 0.0933) mol × 6.022 × 1023 atoms/mol

= 1.124 × 1023 atoms of nitrogen

∴ Number of C atoms in 5.6 g of urea = (1 × 0.0933) mol × 6.022 × 1023 atoms/mol

= 0.562 × 1023 atoms of carbon

∴ Number of O atoms in 5.6 g of urea = (1 × 0.0933) mol × 6.022 × 1023 atoms/mol = 0.562 × 1023 atoms of oxygen.

∴ 5.6 g of urea contain 2.247 × 1023 atoms of H, 1.124 × 1023 atoms of N, 0.562 × 1023 atoms of C and 0.562 × 1023 atoms of O.

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