Question

Which of the following properties is different between two members of a homologous series?
(A) General Formula
(B) Molecular weights
(C) Methods of preparation
(D) Chemical properties

Answer 1

Hint: Homologous series of compounds is the series of organic compounds that have the same functional group and differ from each other only by $$-C{H}_2-$$ units.
e.g. Methane, Ethane, Propane, Butane …..

Step-by-Step Solution:
- As said earlier, homologous compounds have similar functional groups and differ only by $$-C{H}_2-$$ units. So, we can assign them a general formula.
e.g. Homologous series of alkanes have a general formula of $$C_n{H}_{2n+2}$$
Here, by putting numbers like 1,2,3,4…. In place of n gives a homologous series of alkanes.
- As the different compounds of homologous series do not have the same molecular formula, it is obvious that they will not have the same Molecular Weights.
- Now, As the compounds of homologous series contain the same functional group, they can be prepared by the same method of preparation. One just needs to have a related homologous series of reactants in order to obtain a homologous series of products by the same method of preparation.
- As we know that chemical properties of organic compounds are governed by their functional groups, compounds of homologous series will surely have the same chemical properties as they have the same functional groups.
So, based on these reasons, option (B) Molecular weights is correct.

Additional Information:
- General formula is a type of molecular formula in which the number of some atoms is supposed to be “n” and other atoms are represented relatively.
- The Explanation of the fact that homologous series of compounds can be obtained by the same method of preparation is given as below with examples of Esterification Reaction.
$$C{H}_{3}COOH+C{H}_{3}OH\stackrel{con.H_{2}S{O}_4}{\to }C{H}_{3}COOC{H}_3$$
$$C{H}_{3}C{H}_{2}COOH+C{H}_{3}OH\stackrel{con.H_{2}S{O}_4}{\to }C{H}_{3}C{H}_{2}COOC{H}_3$$
$$C{H}_{3}C{H}_{2}C{H}_{2}COOH+C{H}_{3}OH\stackrel{con.H_{2}S{O}_4}{\to }C{H}_{3}C{H}_{2}C{H}_{2}COOC{H}_3$$

Note:
- Here we need not to forget that we are asked about the properties that are NOT the same. Sometimes we make mistakes by assuming the question in a different way.
- In some exceptional cases, Yes, method of preparation of homologous compounds may also differ. But these are just exceptions that are always going to remain present in Chemistry. So, in a broader way there are the same preparation methods available for homologous series of compounds.