Final answer:
The significant obstacle when using osmometry to determine molar masses for compounds with very high molar masses is that the osmotic pressures may be too high to measure.
Step-by-step explanation:
The significant obstacle when using osmometry to determine molar masses for compounds with very high molar masses is that the osmotic pressures may be too high to measure. Osmotic pressure is directly proportional to the number of solute species present in a given amount of solution. For compounds with very high molar masses, the osmotic pressures can become extremely high, making it difficult to accurately measure.