Video Transcript
What is the molecular formula of propanoic acid? (A) C2H3COOH, (B) C2H4COOH, (C) C2H5COOH, (D) C2H6COOH, or (E) C3H5COOH.
To answer this question, let’s start by drawing the structure of propanoic acid. The ending -oic acid indicates that this molecule is a carboxylic acid. All carboxylic acids contain at least one carboxyl group, a carbon atom double bonded to an oxygen atom and single bonded to a hydroxy group. So we can begin to draw the structure of propanoic acid by drawing a carboxyl group.
Returning to the name, the prefix prop- indicates that the molecule contains three carbon atoms. The carboxyl group we drew has one carbon atom. So we know that the structure must contain two additional carbon atoms. The letters in between the prefix prop- and the suffix -oic acid indicate how the carbon atoms are bonded together. A-N tells us that the carbon atoms are joined together by single bonds. So, in our structure, we can single-bond the two carbon atoms in a chain to the carboxyl group. We can complete the structure by adding carbon-hydrogen single bonds until each carbon atom has a total of four bonds.
Now that we know the structure of propanoic acid, we can work out its molecular formula. The molecular formula of a carboxylic acid is often divided into atoms which belong to the R group and atoms which belong to the carboxyl group. The R group of propanoic acid contains two carbon atoms and five hydrogen atoms. The carboxyl group consists of one carbon atom, two oxygen atoms, and one hydrogen atom.
While we could represent the molecular formula of propanoic acid in this way, we are more likely to see the two oxygen atoms written separately to make a distinction between the double-bonded oxygen atom and the hydroxy group. We can now compare our molecular formula to the answer choices. We can see that the correct molecular formula of propanoic acid is answer choice (C) C2H5COOH.
