Understanding the Problem
To determine how many structural isomers can exist for ketones with a molar mass of 100 g/mol, we first need to consider the molecular formula of ketones. Ketones have the general formula \(C_nH_{2n}O\).
Given that the molar mass is 100 g/mol, we can deduce the possible values of \(n\) to find the corresponding molecular formula.

Finding the Molecular Formula
- The molar mass of a ketone can be expressed as:
\[
12n + (2n) + 16 = 100
\]
- Simplifying this equation:
\[
14n + 16 = 100
\]
- Solving for \(n\):
\[
14n = 84 \quad \Rightarrow \quad n = 6
\]
- Thus, the molecular formula for the ketones is \(C_6H_{12}O\).

Identifying the Structural Isomers
The next step is to identify the structural isomers of \(C_6H_{12}O\) that are ketones. Ketones must have a carbonyl group (C=O) between two carbon atoms.
- **Isomer Count**: For \(C_6H_{12}O\):
- The carbon skeleton can be branched in various ways while ensuring that the carbonyl group is placed between carbons.
- The following structural isomers are possible:
- 2-Hexanone
- 3-Hexanone
- 4-Methyl-2-pentanone
- 3-Methyl-3-pentanone
- 2-Pentanone
- 2,2-Dimethyl-3-butanone
- **Total Isomers**: This results in **6 distinct structural isomers**.

Conclusion
Thus, the number of structural isomers for ketones with a molar mass of 100 g/mol is indeed **6**. Therefore, the correct answer is option **d) 6**.