Calculate the weight/mass of :

(a) an atom of oxygen

(b) an atom of hydrogen

(c) a molecule of NH₃

(d) 10²² atoms of carbon

(e) 0.25 gram atom of calcium

(a) Number of oxygen atoms in 16 g of atomic oxygen = 6.022 × 10²³ atoms

∴ mass of 1 atom of oxygen

= $\dfrac{16}{6.022 \times 10^{23}}$

= 2.657 × 10^-23 g

(b) Number of hydrogen atoms in 1 g of atomic hydrogen = 6.022 × 10²³ atoms

∴ Mass of 1 atom of hydrogen

= $\dfrac{1}{6.022 \times 10^{23}}$

= 1.666 × 10^-24 g

(c) Gram molecular mass of NH₃ = 14 + 3 = 17 g

Number of NH₃ molecules in 17 g of NH₃ = 6.022 × 10²³ molecules

Mass of 6.022 × 10²³ molecules of NH₃ = 17g

∴ Mass of 1 molecule of NH₃ = $\dfrac{17}{6.022 \times 10^{23}}$

= 2.823 × 10^-23 g

(d) Mass of 6.022 × 10²³ atoms of atomic carbon = 12 g

∴ Mass of 10²² atoms of carbon = $\dfrac{12}{6.022 \times 10^{23}} \times 10^{22}$

= 0.2 g

(e) Atomic weight of calcium = 40 g

Gram atom = $\dfrac{\text{Mass of element}}{\text{Atomic mass}}$

Therefore, 0.25 = $\dfrac{\text{Mass of calcium}}{40}$

Mass of calcium = 40 x 0.25 = 10 g