Pycnometer determination of specific gravity of soil solids: A 500 cc pycnometer has the following masses: empty pycnometer = 125 g, pycnometer + dry soil = 500 g, pycnometer + dry soil + water (filled to top) = 850 g. Assuming pycnometer full of water = 625 g, compute the specific gravity of soil solids Gs.

Introduction / Context:
Specific gravity of soil solids Gs is needed for many phase relationships (unit weights, void ratio, degree of saturation). The pycnometer method provides Gs by comparing masses of soil, water, and their combination within a known volume vessel.

Given Data / Assumptions:

• Wp (empty pycnometer) = 125 g.
• Wp+s (pyc + dry soil) = 500 g.
• Wp+s+w (pyc + dry soil + water to top) = 850 g.
• Wp+w (pyc + water to top) = 625 g (500 g water in a 500 cc pycnometer).

Concept / Approach:
Use Gs = Ws / [ (Wp+w − Wp) − (Wp+s+w − Wp+s) ], where Ws is mass of dry soil and denominator is mass of an equal volume of water displaced by the soil solids. Multiply by 1 (since water unit mass basis) to obtain Gs directly.

Step-by-Step Solution:

Verification / Alternative check:
The value 2.50 lies in the common range (2.60 ± 0.2 for many mineral soils), plausible for silty sands or mixed mineralogy.

Why Other Options Are Wrong:

• 2.00 and 2.25 are low for most mineral soils (closer to organic-rich soils).
• 2.75 is high for typical quartz–feldspar mixtures.

Common Pitfalls:
Using percent instead of absolute masses; forgetting to subtract pycnometer mass; assuming room-temperature water density far from 1 g/cc without correction for standard tests.

Final Answer:
2.50