# Calculate:  (a) the mass of 1.0505 × 10^23 molecules of carbon dioxide (CO2 ).

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Calculate:

(a) the mass of 1.0505 × 1023 molecules of carbon dioxide (CO2 ).

(b) the number of molecules of 0.25 moles of NH3

(c) the formula unit mass of Na2SO3

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

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(a) First we will convert 1.0505 × 1023 molecules of carbon dioxide (CO2 ) into number of moles:

We know, 1 mole = 6.022 × 1023 (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 CO2 (M)

Mass = 0.17 mol × 44g/mol

Mass = 7.48g

Thus, the mass of 1.0505 × 10 23 molecule of CO2 is 7.48g.

(b) Given: Number of moles = 0.25

We know, 1 mole = 6.022 × 1023 (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) × Na

Number of molecules = 0.25mol × 6.022 × 1023

Number of molecules = 1.5 × 1023

Thus, the number of molecules of 0.25 moles of NH3 is 1.5 × 1023

(c) Formula unit mass of Na2SO3

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 Na2SO3 is 126 u