text: # 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