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ICSE Class 10 Chemistry 2016 Solved Paper

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Question : 63
Total: 85
A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29 , find its molecular formula. [C=12,H=1]
Solution:  
 Element / Compound  % Mass  Atomic or Mol. Mass  No. of Atoms  Simplest Ratio  Rounding off Ratio
 C  82.76  12  ‌‌
82.76
12
=6.89		  ‌
6.89
6.89
=1		  1×2=2
 H  17.21  1  ‌‌
17.24
1
=17.24		  ‌
17.24
6.89
=2.5		  2.5×2=5

‌∴‌ The empirical formula is ‌C2H5‌. ‌
Empirical formula mass of C2H5=12×2+5
‌×1=29‌. ‌
‌‌ V.D. ‌=29‌ (given) ‌
‌‌ Mol. Mass ‌=2×‌ V.D. ‌
‌=2×29=58
‌n=‌
‌ Molecular Mass ‌
‌ Emp. formula Mass ‌

‌=‌
58
29
=2
Molecular formula =(‌ Empirical formula ‌)n
‌=(C2H5)2
‌=C4H10
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