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An organic compound with a vapour density 56.5 has the following percentage composition:C =53.1% N...

Chemistry
JAMB 1984

An organic compound with a vapour density 56.5 has the following percentage composition:C =53.1% N = 12.4%, O = 28.3%, H = 6.2%. The molecular formula of the compound is
Relative atomic masses: N = 14, O = 16, C = 12, H = 1 )

  • A. C3H6O2N
  • B. C5H6O2N
  • C. (C5H7O2N)1/2
  • D. C5H7O2N
  • E. (C5H7O2N)2
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Correct Answer: Option D
Explanation

Elements :                                          C                N              O           H

% By mass                                       53.1           12.4          28.3        6.2

Divide through by atomic mass       \(\frac{53.1}{12}\)    \(\frac{12.4}{14}\)         \(\frac{28.3}{16}\)    \(\frac{6.2}{1}\)

                                                          4.425        0.8857     1.7688    6.2

Divide through by the least amount \(\frac{4.425}{0.8857}\)      \(\frac{0.8857}{0.8857}\)       \(\frac{1.7688}{0.8857}\)      \(\frac{6.2}{0.8857}\)

                                                               5      :       1        :   2   :        7

Empirical formula :   C\(_5\)N\(_1\)O\(_2\)H\(_7\)  ⇒     C\(_5\)H\(_7\)NO\(_2\)

 

Recall that (Empirical formula)n = Relative molecular mass 

But Relative molecular mass =  2 x Vapour density

So,             (Empirical formula)n = 2 x Vapour density

                     [C\(_5\)H\(_7\)NO\(_2\) ] n = 2 x 56.5

                   [ (12 x 5) + (1 x 7) + 14 + (16 x 2) ] n = 113

                  [60 + 7 + 14 + 32] n = 113

                         113n = 113

                         n  = \(\frac{113}{113}\) = 1

  [C\(_5\)H\(_7\)NO\(_2\) ] n =  [C\(_5\)H\(_7\)NO\(_2\) ] 1 ⇒   C\(_5\)H\(_7\)NO\(_2\) - Option D


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