Question

What is the molar mass of the third member of the homologous series if the molar mass of the first member is 46 g?

• A. 60 g
• B. 74 g
• C. 138 g
• D. 80 g

Hint:

The molar mass of successive members of a homologous series increases by approximately 14 g/mol.

Answer 1

The Correct Option is B

The problem involves understanding homologous series in chemistry. A homologous series is a group of organic compounds that have a similar structure but differ by a repeating unit, often a CH₂ group (methylene group). Given that the molar mass of the first member is 46 g, we are to find the molar mass of the third member. We'll use the concept of a constant difference in molar mass between consecutive members of a homologous series.

In many homologous series, each successive member differs by 14 g/mol, which is the mass of a CH₂ group. Therefore, this is how we can solve the problem:

1. Identify the increase in molar mass from one member to the next, which is 14 g (molecular weight of CH₂).
2. Calculate the molar mass of the second member: \(46 \, \text{g/mol} + 14 \, \text{g/mol} = 60 \, \text{g/mol}\).
3. Determine the molar mass of the third member: \(60 \, \text{g/mol} + 14 \, \text{g/mol} = 74 \, \text{g/mol}\).

Thus, the molar mass of the third member is 74 g.