After repeating vacuum drying, dry mass determinations within 0.3 g indicate completion.

• A. True
• B. False
• C. They indicate ongoing drying
• D. They indicate contamination

Answer: (A)

In vacuum drying the goal is to reach a constant dry mass, meaning all moisture has been removed and the mass stops changing with further drying. After a drying cycle, the sample is weighed again; if the mass difference between successive dryings is within 0.3 g, that small change is within the balance’s precision and the variability of the sample, so you can treat the mass as final. If the difference were larger, more drying would be needed because moisture could still be present. This tolerance balances accuracy with practicality, ensuring that the recorded dry mass truly reflects a dry sample rather than a fluctuating measurement. So, achieving dry masses within 0.3 g indicates completion.