A compound with the empirical formula CH2 has a molar mass of 112 g/mol. What is the molecular formula for this compound?

tiburtius | High School Teacher | (Level 2) Educator

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Empirical formula gives us ratio of the number of atoms in a compound. In this case we only know that ratio of carbon atoms to hydrogen atoms is 1:2 but we don't know the exact number of atoms in the compound. Thus the molecular formula could be CH_2 (unlikely because that would leave carbon with two unpaired electrons), C_2H_4, C_3H_6 etc. or more generally C_nH_(2n).

To get the exact molecular formula we must use molar mass of the compound. To get molar mass of a compound we simply add molar masses of all the atoms in the compound.

Now we calculate molar mass of CH_2 :

M(CH_2)=12.011+2\cdot1.008=14.027 g mol^(-1)

To get n from our molecular formula we simply divide molar mass of compound 112 by molar mass we obtained 14.027.

n=112/(14.027)=7.985approx8

Now we just insert the obtained n into the molecular formula above. Hence, the molecular formula of the compound is C_8H_16.

Note: Molar mass of an atom is atomic mass of an element (which you can read from the table of elements) multiplied by 1 g/mol (e.g. molar mass of an hydrogen atom is 1.008 g/mol).

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