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 = 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:

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 = 0.5

We know, 1 mole = 6.022 × 10²³(N_a = 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