Which statement is true about weighing for small pycnometer use?

• A. When using the small pycnometer, the fixture is weighed together with the pycnometer.
• B. The fixture is weighed separately from the pycnometer.
• C. The small pycnometer should not be weighed at all.
• D. Weighing is optional for the small pycnometer.

Answer: (A)

Weighing the fixture together with the small pycnometer is essential because the mass used in the density calculations must reflect the entire assembly that will interact with the sample and the water during the test. When you determine the mass in air and the mass when the pycnometer is filled with water, you’re relying on the total weight of the device as it actually appears in the measurement. The fixture is part of that setup, so including it in the tare/initial weight ensures the calculations are accurate and consistent from one test to the next.

If you tried to weigh the fixture separately, you would introduce extra steps and potential errors in subtracting the fixture’s mass from the pycnometer’s mass, plus any small changes in seating or contact could affect the reading. The procedure is designed so the combined weight is the reference, making the process straightforward and reliable. Therefore, the fixture is weighed together with the pycnometer.