Question

Deduce the number of molecules of the following compounds in the given quantities.

(i) 32g oxygen, (ii) 90g water, (iii) 8.8g carbon dioxide, (iv) 7.1g chlorine.

Answer 1

(i) 32 g of oxygen :
Molecular mass of O₂ = 2x16=32

Number of moles of O₂ = Mass of O₂ in grams/Molecular mass of O₂ = 32/32 = 1mol                    
∴ 1 mole of O₂ contains-----6.022 x 10²³ molecules
∴ 32g of oxygen contains 6.022 x 10²³ molecules

(ii) 90 g of water :
Molecular mass of water H₂O= 2x(1)+16 =18

Number of moles in water = Mass of water in grams/Molecular mass of water = 90/18 = 5mole

∴ 5 moles of H₂O contains-----5 x 6.022 x 10²³ molecules = 30.11 x  10²³ molecules

 ∴ 90g of water contains 30.11 x  10²³ molecules

(iii) 8.8 g of CO₂ :

Molecular mass of Carbon dioxide CO₂ =12+2x(16) =44
Number of moles in CO₂ = Mass of CO₂ in grams / Molecular mass of CO₂ = 8.8/44 = 0.2mol

  ∴0.2 moles of CO₂ contains-----0.2 x 6.022 x 10²³ molecules = 1.2044 x 10²³ molecules 

∴8.8 g of CO₂ contains 1.2044 x  10²³ molecules

(iv) 7.1 g of chlorine
Molecular mass of Chlorine Cl₂ = 2x(35.5)=71
  No. of moles in chlorine=Mass of chlorine in grams/Molecular mass of chlorine  =7.1/71 = 0.1mol

∴0.1 moles of Cl₂ contains-----0.1 x 6.022 x 10²³ molecules = 0.6022 x 10²³ molecules

∴ 7.1 g of chlorine contains 0.6022 x 10²³ molecules