Relative dissolved oxygen (DO): Compared with fresh river water at the same temperature and pressure, seawater typically contains how much dissolved oxygen?
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AAbout 10% less DO
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BAbout 20% less DO
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CAbout 10% more DO
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DAbout 20% more DO
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EAbout 5% less DO
Answer
Correct Answer: About 20% less DO
Explanation
Introduction / Context:Dissolved oxygen (DO) solubility guides aeration, ecology, and outfall design. Salinity reduces oxygen solubility relative to freshwater, which matters when estimating reaeration and oxygen sag in marine discharges.
Given Data / Assumptions:
- Equal temperature and barometric pressure for comparison.
- Typical ocean salinity (~35 g/L).
- Focus on equilibrium DO solubility.
Concept / Approach:
Gas solubility decreases with increasing salinity (the “salting out” effect). Empirical correlations and tables show seawater DO solubility is roughly 15–25% lower than that of freshwater over common environmental temperatures, making “about 20% less” a practical rule of thumb.
Step-by-Step Solution:
Hold temperature and pressure constant to isolate salinity effect.Consult typical solubility differences; seawater shows substantial reduction.Choose the closest rounded value: about 20% less.Verification / Alternative check:
Standard DO tables (fresh vs. sea) confirm the reduction near 20% across common temperatures.
Why Other Options Are Wrong:
10% less underestimates the salinity effect; “more DO” contradicts the salting-out principle; 5% less is too small.
Common Pitfalls:
Comparing DO concentration measurements influenced by biological demand rather than equilibrium solubility; ignoring temperature/pressure dependence.
Final Answer:
About 20% less DO