Video: Applying the General Formula for Alkane Chemical Formulas

What is the molecular formula of an alkane with 7 carbon atoms?

02:06

Video Transcript

What is the molecular formula of an alkane with seven carbon atoms?

There are three bits of information in this question. I’ll go through them one by one. The first is a molecular formula. What exactly is it? A molecular formula gives the total number of atoms of each element in one molecule of a substance. So for a substance such as propanone, the molecular formula shows the number of carbon atoms, three, the number of hydrogen atoms, six, and the number of oxygen atoms, one. So this is the format we’re looking for for our answer.

The second piece of information in the question is that we’re after the formula of an alkane. An alkane is a saturated hydrocarbon. This means that it’s an organic compound that contains no carbon–carbon double or triple bonds, which contains only carbon and hydrogen atoms. The last piece of information in the question is that there are seven carbon atoms in this alkane. This means that we know that the molecular formula for this alkane will start C₇. We also know that since an alkane is a hydrocarbon that it will contain hydrogen. We just don’t know how much.

The general formula for an alkane without a ring is C_(𝑛) H_(2𝑛 + 2). If 𝑛 is equal to seven, two 𝑛 plus two is equal to two times seven plus two, which is equal to 14 plus two, which is equal to 16. So the number of hydrogen atoms in our molecular formula is 16. Alternatively, you could’ve just drawn out structure for alkane with seven carbon atoms and counted the number of hydrogens as 16.

Either way, we’ve demonstrated that the molecular formula of an alkane with seven carbon atoms is C₇H₁₆.

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