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Question
The gases hydrogen, oxygen, carbon dioxide, sulphur dioxide and chlorine are arranged in order of their increasing relative molecular mass. Given 8 g of each gas at STP, which gas will contain the least number of molecules and which gas the
most?
Solution 1
Calculation of number of molecules in each gas sample :
(i) 8g of hydrogen :
1g of hydrogen = 6.023 x 1023 molecules
8 g of hydrogen will have = 6 .023 x 1023x 8 = 48 .184 x 1023 molecules
(ii ) 8 g of oxygen :
32 g of oxygen has = 6 .023 x 1023 molecules
8 g of oxygen will have = 6 .023 x 1023/ 32 x 8 = 1.50 x1023 molecules
(iii ) 8 g of carbon dioxide :
44 g of carbon dioxide has = 6 .023 x 1023 molecules
8 g of carbon dioxide will have = 6 .023 x 1023/ 44 x 8 = 1.0 x1023 molecules
(iv) 8 g of sulphur dioxide :
64 of sulphur dioxide has = 6 .023 x 1023 molecules
8 g of sulphur dioxide will have = 6 .023 x 1023/ 64 x 8 = O. 75 x l 023
molecules
(v) 8 g of chlorine :
35 .5 g of chlorine has = 6 .023 x 1023 molecules
8 g of chlorine will have = 6 .023 x 1023/ 71 x8 = 0 .68 x1023 molecules
Hence, chlorine gas will have least number of molecules. Hydrogen gas will have
the most number of molecules.
Solution 2
Calculation of number of molecules in each gas sample :
(i) 8g of hydrogen :
1g of hydrogen = 6.023 x 1023 molecules
8 g of hydrogen will have = 6.023 x 1023 x 8 = 48.184 x 1023 molecules.
(ii) 8g of oxygen :
32g of oxygen has = 6.023 x 1023 molecules
8g of oxygen will have = 6.023 x 1023/ 32 x 8 = 1.50 x1023 molecules
(iii) 8g of carbon dioxide :
44g of carbon dioxide has = 6.023 x 1023 molecules
8 g of carbon dioxide will have = 6 .023 x 1023/ 44 x 8 = 1.0 x1023 molecules
(iv) 8g of sulphur dioxide :
64 of sulphur dioxide has = 6.023 x 1023 molecules
8g of sulphur dioxide will have = 6.023 x 1023/ 64 x 8 = O. 75 x 1023 molecules
(v) 8g of chlorine :
35.5 g of chlorine has = 6.023 x 1023 molecules
8g of chlorine will have = 6.023 x 1023/ 71 x 8
= 0.68 x1023 molecules
Hence, chlorine gas will have least number of molecules. Hydrogen gas will have the most number of molecules.
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