Calculate the number of atoms in each of the following : i. 64 u of oxygen (O) ii. 42 g of nitrogen (N)

i. 64 u of oxygen (O) = x atoms 

Atomic mass of oxygen (O) = 16 u 

∴ Mass of one oxygen atom = 16 u

∴ x = \(\frac{64u}{16u}\)

= 4 atoms

ii. 42 g of nitrogen (N) 

Atomic mass of nitrogen = 14 u 

∴ Molar mass of nitrogen = 14 g mol^-1

Now,

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

= \(\frac{42g}{14g\,mol^{-1}}\) 

= 3 mol

Now, 

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

= 3 mol × 6.022 × 10²² atoms/mol

= 18.07 × 10²³ atoms

= 1.807 × 10²⁴ atoms

∴ i. Number of oxygen atoms in 64 u = 4 atoms

ii. Number of nitrogen atoms in 42 g = 1.807 × 10²⁴ atoms