Question 11:
From the following data, identify the correct pH value of seawater:
Seawater typically has a pH around 8.1, making option (C) 7.8 the closest and correct choice among the given alternatives.
Correct Answer: (C) 7.8
Seawater’s average surface pH is approximately 8.1-8.2, buffered by carbonates and bicarbonates, though it ranges 7.5-8.4 due to salinity and depth. Recent ocean acidification from CO₂ absorption has lowered it slightly to ~8.05-8.1, but 7.8 best matches typical values over neutral 7.0.
Options Analysis
| Option | pH Value | Explanation |
|---|---|---|
| (A) | 6.5 | Acidic; fits polluted freshwater or acidified deep waters, not standard seawater (always >7.5). |
| (B) | 7.0 | Neutral (pure water pH); seawater is alkaline due to salts like NaHCO₃, not neutral. |
| (C) | 7.8 | Correct; lower end of surface range (7.8-8.4), accounts for variability and acidification trends. |
| (D) | 8.5 | Slightly high; possible in high-salinity or productive surface areas, but exceeds average ~8.1. |
Influencing Factors
High alkalinity (~116 mg/L as CaCO₃) buffers pH against changes; CO₂ dissolution forms carbonic acid, dropping pH ~0.1 units since pre-industrial 8.2. For exams, note seawater’s slight alkalinity supports marine life like corals (optimal >7.6).
