Question

(i) Calculate the formula unit mass of Na₂CO₃ . 

[given atomic mass of Na = 23 u, C = 12 u and O = 16 u]

(ii) What is the mass of one mole of oxygen atoms?

(iii) Convert 12g of oxygen gas into mole, if atomic mass of oxygen is 16 u

Answer 1

(i) Formula unit mass of Na₂CO₃ :

2 × (Atomic mass of Na) + atomic mass of C + 3 × (Atomic mass of oxygen) 

⇒ 2 × 23 + 12 + 3×16 

⇒ 46 + 12 + 48 

⇒ 106u 

(ii) To calculate the mass of 1 mole of oxygen atoms: 

By applying the formula given:

Number of moles = \(\frac{Mass\,of\,oxygen\,atoms}{Molar\,mass\,of\,oxygen}\)

Mass of oxygen atoms = Number of moles × Molar mass 

⇒ Mass of oxygen atoms = 1mole × 32g/mol 

⇒ Mass of oxygen atoms = 32g 

Thus, the mass of 1 mole of oxygen atoms is 32g. 

(iii) Given: Mass of oxygen = 12g 

Atomic mass = 16u 

By applying the formula given:

Number of moles = \(\frac{Mass\,of\,oxygen\,atoms}{Molar\,mass\,of\,oxygen}\) 

⇒ Number of moles = \(\frac{12g}{16u}\) 

⇒ Number of moles = 0.75mol 

Thus, the number of moles = 0.75mol