Question

Calculate the number of atoms of ‘C’, ‘H’ and ‘O’ in 72.5 g of isopropanol, C₃H₇OH (molar mass 60 g mol^-1).

Answer 1

Mass of isopropanol(C₃H₇OH) = 72.5 g

The number of atoms of C, H, O

Calculation : 

Molecular formula of isopropanol, is C₃H₇OH.

Molar mass of C₃H₇OH = 60 g mol^-1

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

= \(\frac{72.5g}{60g\,mol^{-1}}\) 

= 1.208 mol 

∴ Moles of isopropanol = 1.21 mol 

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

Now, 

1 molecule of isopropanol has total 12 atoms, out of which 8 atoms are of H, 3 of C and 1 of O.

∴ Number of C atoms in 72.5 g isopropanol = (3 × 1.208) mol × 6.022 × 10²³ atoms/mol 

= 2.182 × 10²⁴ atoms of carbon

∴ Number of ‘H’ atoms in 72.5 g isopropanol

= (8 × 1.208) mol × 6.022 × 10²³ atoms/mol

= 5.819 × 10²⁴ atoms of hydrogen

∴ Number of ‘O’ atoms in 72.5 g isopropanol 

= (1 × 1.208) mol × 6.022 × 10²³ atoms/mol 

= 7.274 × 10²³ atoms of oxygen

∴ 72.5g of isopropanol contain 2.182 ×10²⁴ atoms of H and 7.274 × 10²³ atoms of O.