Video Transcript
An unidentified hydrocarbon is
cracked. It produces two product
molecules. One of these molecules has the
chemical formula C10H22. The other one has the formula
C2H4. What is the formula of the
unidentified hydrocarbon? (A) C11H22, (B) C11H24, (C) C12H24,
(D) C12H26, or (E) C13H28.
The hydrocarbon whose formula we’re
trying to determine has undergone a cracking reaction. A cracking reaction is a type of
decomposition reaction where larger organic molecules are broken down into smaller
molecules. Typically, overall when a larger
hydrocarbon is cracked, carbon-to-carbon bonds are broken so that smaller organic
molecules can be produced. The cracking reaction in the
question resulted in two products: C10H22 and C2H4.
We know that C10H22 is an alkane,
as it fits the general formula of alkanes of C𝑛H2𝑛+2, where 𝑛 in this case is
equal to 10. We also know that C2H4 is an
alkene, as it fits the general formula of alkenes, which is C𝑛H2𝑛, where in this
case 𝑛 is equal to two. Let’s use the chemical formulas of
the products to deduce the chemical formula of the unidentified hydrocarbon.
In a cracking reaction, just as we
would expect in any chemical reaction, the total number of each type of atom present
in the reactants must be equal to the total number of each type of atom present in
the products. We can calculate the total number
of carbon and hydrogen atoms present after the cracking reaction occurred to find
the number of carbon and hydrogen atoms originally present in the unidentified
hydrocarbon. The total number of carbon atoms in
the products would be 10 plus two, which gives us a total of 12. The total number of hydrogen atoms
in the products would be 22 plus four, which gives us 26. So there must be 12 carbon atoms
and 26 hydrogen atoms in the reactants.
Since there is only one reactant
molecule in this cracking reaction, we can use these values as the subscripts in the
chemical formula, which gives us C12H26. Therefore, the unidentified
hydrocarbon that underwent the cracking reaction has the chemical formula in answer
choice (D), C12H26.
