SEA-WATER CHEMISTRY — Relative abundance of major ions Question: Select the correct descending order of the relative abundance of dissolved salts (major ions) in average seawater.

Introduction / Context:
The major ions in seawater occur in remarkably constant proportions, a principle known as constancy of composition. Knowing their relative abundances supports understanding of ocean chemistry, salinity, and biogeochemical cycles.

Given Data / Assumptions:

• We consider the six most abundant ions in average open-ocean water.
• Units are in mass or equivalent proportions; the descending order is asked.
• Minor regional variations do not change the basic global pattern.

Concept / Approach:
The typical order by abundance is: chloride (as Cl⁻) first, then sodium (Na⁺), followed by sulfate (SO₄²⁻), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺). This ordering reflects input sources, long residence times, and removal mechanisms.

Step-by-Step Solution:
Recall the canonical major-ion list and order.Compare each option against the canonical order.Option (a) exactly matches the standard descending sequence.Therefore, select (a).

Verification / Alternative check:
Oceanography texts list approximate mass percentages: Cl⁻ ~55%, Na⁺ ~31%, SO₄²⁻ ~7.7%, Mg²⁺ ~3.7%, Ca²⁺ ~1.2%, K⁺ ~1.1, consistent with this order.

Why Other Options Are Wrong:
(b), (c), (d), (e) rearrange ions into orders that contradict the established ranking (e.g., placing magnesium ahead of sulfate or sodium ahead of chloride).

Common Pitfalls:
Mixing up similar-sounding ions or assuming sodium is first because table salt is NaCl; the mass contribution from chloride exceeds sodium in seawater.

Final Answer:
chlorine, sodium, sulphate, magnesium, calcium, potassium