Experimentally determined molar mass is always lower than the value in case of :

Strong electrolyte

The correct answer is option 1. Strong electrolyte.

When determining molar mass experimentally using colligative properties (like boiling point elevation or freezing point depression), we assume that the solute particles behave ideally in solution. However, this assumption breaks down for electrolytes, which dissociate into ions in solution.

Strong electrolytes: These completely dissociate into their constituent ions, leading to a greater number of particles in solution than expected based on the original number of molecules. This results in a lower experimentally determined molar mass compared to the theoretical value.

Weak electrolytes: These only partially dissociate, so the increase in the number of particles is less significant. Therefore, the experimentally determined molar mass is closer to the theoretical value.

Non-electrolytes: These do not dissociate at all, so the number of particles in solution remains the same. The experimentally determined molar mass will be close to the theoretical value.

Dimers: These are molecules that associate to form larger molecules. This leads to a lower number of particles in solution than expected, resulting in a higher experimentally determined molar mass compared to the theoretical value.

Therefore, the only case where the experimentally determined molar mass is consistently lower than the theoretical value is for strong electrolytes.