Question

Calculate: 

(a) The number of moles of Sulphur (S₈ ) present in 16 g of solid sulphur. 

(b) the mass of 10 moles of sodium sulphite (Na₂SO₃). 

(c) the number of atoms in 11.5 g of Na. 

[atomic mass: Na = 23u; S = 32u, O = 16u, NA = 6.022 × 10²³ mol ^–1 ]

Answer 1

a) Given: 

Mass of sulphur (m) = 16g 

Atomic mass of sulphur (S₈ ) = 8 × 32u = 256 u 

Molar mass of S₈ (M) = 256g/mol 

To calculate the number of moles, we will apply the formula given below: 

Number of moles = \(\frac{Given\,mass(m)}{Molar\,mass(M)}\)

Number of moles = \(\frac{16g}{256g/mol}\)

Number of moles = 0.0625 

Thus, the number of moles in 16 g of solid sulphur is 0.0625.

b) Given: 

Number of moles = 10 Molecular mass of sodium sulphite (Na₂SO₃ ) = 2 × Atomic mass of Na + atomic mass of sulphur + 3 × Atomic mass of O 

= 2 × 23 u + 32 u + 48 u 

= 126 u 

Molar mass of Na₂SO₃ = 126g/mol 

To calculate the mass, apply the formula:

Number of moles = \(\frac{Mass(m)}{Molar\,mass(M)}\)

We can write: 

Mass = Number of moles × Molar mass 

Mass = 10 mol × 126g/mol 

Mass = 1260g 

Thus, the mass of sodium sulphite is 1260g.

c) Given: 

Mass of Na (m) = 11.5g 

Atomic mass of sulphur = 23u 

Molar mass of sulphur (M) = 23g/mol 

To calculate the number of moles, we will apply the formula given below:

Number of moles = \(\frac{Given\,mass(m)}{Molar\,mass(M)}\)

Number of moles = \(\frac{11.5g}{23g/mol}\)

Number of moles = 0.5

We know, 1 mole = 6.022 x 10²³(N_a = Avogadro's number)

The number of moles = \(\frac{Number\,of\,atoms}{Avogadro's\,number}\)

We can write, number of atoms = Number of moles × N_a 

Number of atoms = 0.5 × 6.022 × 10²³ 

Number of atoms = 3.022 × 10²³ 

Thus, the number of atoms in 11.5 g of Na is 3.022 × 10²³