Question Video: Determining the General Formula of a Compound from a Displayed Formula | Nagwa Question Video: Determining the General Formula of a Compound from a Displayed Formula | Nagwa

Question Video: Determining the General Formula of a Compound from a Displayed Formula Chemistry • Third Year of Secondary School

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What is the general formula for the group of the compounds to which the displayed formula is related?

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Video Transcript

What is the general formula for the group of the compounds to which the following displayed formula is related?

In this question, we are given the displayed formula of a molecule. A displayed formula is a representation of a molecule showing all the atoms and bonds, and the bonds are drawn as lines. When looking at the provided displayed formula, we can see that the molecule contains only carbon and hydrogen atoms. Therefore, we can classify this molecule as a hydrocarbon. We also notice that the molecule contains one carbon-to-carbon triple bond. Hydrocarbons that contain at least one double or triple bond between carbon atoms are classified as unsaturated hydrocarbons. And the type of unsaturated hydrocarbon that contains one or more triple carbon-to-carbon bonds is called an alkyne.

So we’ve determined that the molecule belongs to a class of hydrocarbons called alkynes. At this point, it would be helpful for us to write a molecular formula for the given molecule. A molecular formula is a chemical formula expressing the exact number and type of atoms of each element in a molecule. To write a molecular formula, we will need to count up the total number of carbon and hydrogen atoms in the molecule. According to the displayed formula, there are a total of six carbon atoms in the molecule and there are a total of 10 hydrogen atoms. In a molecular formula, the subscripts to the right of each chemical symbol represent the number of that type of atom in the molecule. Therefore, the molecular formula of the given molecule is C6H10.

Now, the question is asking us to find a general formula for the group of compounds that the given molecule belongs to. So we need to know the general formula of an alkyne. The general formula of an alkyne is C𝑛H2𝑛−2. In this formula, 𝑛 represents the number of carbon atoms in the alkyne molecule. The alkyne given in the problem contains six carbon atoms. Therefore, when using the general formula, 𝑛 is equal to six. After substitution, we get C6H2(6)−2. After simplifying, we get the expected molecular formula, C6H10. The general formula of an alkyne represents the molecular formula of any alkyne, such as the one in this problem.

Let’s demonstrate this relationship by looking at the two simplest alkynes: ethyne, or acetylene, and propyne. According to the displayed formula, the ethyne molecule contains two carbon atoms. Therefore, we will need to substitute two for 𝑛 in the general formula. After substitution, we get C2H2(2)−2. After simplification, we get C2H2, which is the correct molecular formula for ethyne. The propyne molecule contains three carbon atoms. Therefore, we will need to substitute three for 𝑛 in the general formula for an alkyne. This gives us C3H2(3)−2. After simplification, we get C3H4, which is the correct molecular formula for the propyne molecule.

In conclusion, the general formula for the group of the compounds to which the following displayed formula is related is C𝑛H2𝑛−2.

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