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9th Class Science Structure of the Atom Question Bank Structure of Atom

  • question_answer
    A compound of carbon, hydrogen and nitrogen contains these elements in the ratio 9 : 1 : 3.5. If its molecular mass is 108, what is the molecular formula?

    A) \[{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}\text{N}\]                      

    B) \[{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{4}}}\text{N}\]          

    C)        \[{{\text{C}}_{\text{2}}}\text{H}{{\text{N}}_{\text{2}}}\]          

    D)        \[{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{8}}}{{\text{N}}_{\text{2}}}\]                   

    Correct Answer: D

    Solution :

    Ratio of elements C, H and N = 9 : 1 : 3.5 Whole number ratio of elements C, H and N=18:2:7
    Element Ratio Atomic mass Atomic ratio Simplest atomic ratio
    C 18 12 \[\frac{18}{12}=1.5\] \[1.5\times 2=3\]
    H 2 1 \[\frac{2}{1}=2\] \[2\times 2=4\]
    N 7 14 \[\frac{\text{7}}{\text{14}}\text{=0}\text{.5}\] \[\text{0}\text{.5}\times \text{2=1}\]
    Thus, empirical formula of the compound \[\text{=}{{\text{C}}_{3}}{{\text{H}}_{\text{4}}}\text{N}\] Empirical formula mass of the compound \[\text{=12}\times \text{3+1}\times \text{4+1}\times \text{14=36+4+14=54}\] \[n=\frac{\text{Molecular mass}}{\text{Empirical formula mass}}=\frac{108}{54}=2\] Molecular formula\[=n\times \]empirical formula             \[\text{=(}{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{4}}}\text{N}{{\text{)}}_{\text{2}}}\text{=}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{8}}}{{\text{N}}_{\text{2}}}\]


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