Final answer:
To fix a bubble, it needs to be caught when the last bit of liquid becomes gas, sealed, and then allowed to condense back into a liquid for mass determination.
Step-by-step explanation:
In order for a bubble to be fixable, it needs to be caught at the moment when the last bit of liquid in the bubble becomes gas. At this point, the bubble will be filled with only gaseous sample at ambient pressure. The flask can then be sealed and the gaseous sample can condense back into a liquid, allowing the sample's mass to be determined.