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Question

A compound having molecular mass 30 g/mol is found to contain 80% of carbon and 20% of hydrogen, then the molecular formula of the compound is:

A
C6H6
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B
C2H5OH
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C
C2H6
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D
C2H4
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Solution

The correct option is C C2H6
Let the mass of unknown compound be 100 g.
Given that the compound contains 80% of carbon and 20% of hydrogen.
Therefore, weight of carbon in the compound =80g
weight of hydrogen in the compound =20g
Molecular weight of carbon =12g
Molecular weight of hydrogen =1g
moles of carbon in the compound =8012=6.67
Moles of hydrogen in the compound =201=20
Ratio of no. of moles of carbon to no. of moles of hydrogen-
No. of moles of carbon : no. of moles of hydrogen =6.67:201:3
3 moles of hydrogen are present in the compound for 1 mole of carbon.
Hence, the empirical formula of the compound is CH3.
empirical mass=15 g/mol
Molecular formula =(CH3)n
whereas,
n=molecular massempirical mass=3015=2
Thus, the molecular formula of the compoud is C2H6

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