**(a) First we will convert 1.0505 × 10**^{23} molecules of carbon dioxide (CO_{2} ) into number of moles:

We know, 1 mole = 6.022 × 10^{23} (Na = Avogadro’s number)

The number of moles = \(\frac{Given\,number\,of\,molecules}{Avogadro's\,number}\)

Number of moles = \(\frac{1.0505\times10^{23}}{6.022\times10^{23}}\)

Number of moles = 0.17

**Now, to calculate mass, apply the formula:**

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

We can write:

Mass(m) = Number of moles (n) × Molar mass of CO_{2} (M)

Mass = 0.17 mol × 44g/mol

Mass = 7.48g

**Thus, the mass of 1.0505 × 10 23 molecule of CO**_{2} is 7.48g.

**(b)** Given: Number of moles = 0.25

We know, 1 mole = 6.022 × 10^{23 }(Na = Avogadro’s number)

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

We can write:

Number of molecules = Number of moles (n) × N_{a}

Number of molecules = 0.25mol × 6.022 × 10^{23 }

Number of molecules = 1.5 × 10^{23}

**Thus, the number of molecules of 0.25 moles of NH**_{3} is 1.5 × 10^{23 }

**(c) **Formula unit mass of Na_{2}SO_{3} :

2 × Atomic mass of Na + Atomic mass of sulphur + 3 × Atomic mass of O

= 2 × 23 u + 32 u + 3 × 16 u

= 126 u

**Thus, the formula unit mass of Na**_{2}SO_{3} is 126 u