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]


(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)




Number of moles = 0.17


Now, to calculate mass, apply the formula:




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 × 1023 molecule of CO2 is 7.48g.


(b) Given: Number of moles = 0.25


We know, 1 mole = 6.022 × 1023(Na = 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


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