Video Transcript
What is the common molecular formula for the following collection of molecules?
To begin, the six molecules shown are organic molecules. Now, it may be a bit confusing to see structural formulas presented using mostly lines. Where are the carbon and hydrogen atoms that we know must be present in these molecules? Let’s take a look at the structural formula at the top left.
Each line segment in the structural formula represents a single covalent bond between atoms. Let’s use pink circles to represent the carbon atoms in the molecule. Carbon atoms are located at the points at which two line segments meet. Therefore, this molecule contains a total of four carbon atoms. We notice that in this structural formula, hydrogen atoms that are bonded to the carbon atoms are not shown. Let’s draw a displayed formula to help us.
We can determine that the molecule must contain a total of eight hydrogen atoms, which ensures that each carbon atom is surrounded by a total of four bonds. Finally, we know that there is also one oxygen atom present in the molecule. Now, we’re ready to write a molecular formula for the molecule, which is a chemical formula that expresses the exact type and number of atoms of each element in the molecule.
The molecular formula of this molecule is C4H8O. Now, we need to remember that this problem told us that the molecular formula of all six molecules is the same. This must mean that these molecules are structural isomers, which are molecules with the same molecular formula that differ from one another by how their atoms are connected. We can probably see this most clearly by noticing the variety of ring shapes. In some of the molecules, the oxygen atom is part of the ring structure, and in others, it is not.
We can probably recognize that the top two molecules are alcohols because they contain the hydroxy or OH group. The remaining four molecules are ethers. In an ether linkage, an oxygen atom has two single bonds to adjacent carbon atoms. It’s almost difficult to believe that all six of these molecules could have the same molecular formula. Just to be sure, let’s confirm that this is true by identifying the number and type of each atom within one more molecule. Moving down to the next molecule on the left-hand side, we notice there are three carbon atoms at the vertices in the ring structure. There is an additional carbon atom at the end of the line segment, extending from the oxygen atom.
As expected, there are a total of four carbon atoms in the molecule. Drawing the displayed formula helps us to determine that there are a total of eight hydrogen atoms in the molecule. As we can see, each carbon atom is surrounded by a total of four bonds. Finally, there is one oxygen atom in the molecule. The molecular formula of this molecule is therefore C4H8O.
Now that we know how to interpret this type of structural formula, we can see that the remaining molecules have four carbon atoms each. They also contain one oxygen atom and eight hydrogen atoms each. In conclusion, the common molecular formula for the collection of molecules shown is C4H8O.
