Water quality chemistry – identify the incorrect statement about pH and hydrogen-ion concentration Which of the following statements is incorrect regarding the pH scale and hydrogen-ion concentration in water?
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AThe pH value indicates the logarithm of the reciprocal of hydrogen-ion concentration
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BHigher pH corresponds to lower hydrogen-ion concentration
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CLower pH corresponds to higher hydrogen-ion concentration
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DLower pH gives an alkaline solution
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ENone of these
Answer
Correct Answer: Lower pH gives an alkaline solution
Explanation
Introduction / Context:pH is a master variable for water treatment, corrosion control, and disinfection. Correct interpretation of pH relative to acidity and alkalinity is fundamental to process decisions.
Given Data / Assumptions:
- pH is defined as pH = −log10[H+], which equals log10(1/[H+]).
- Neutral water at 25°C has pH ≈ 7.
- Acidic solutions have pH < 7; alkaline (basic) solutions have pH > 7.
Concept / Approach:Because pH is the negative logarithm, an increase of pH by 1 indicates a tenfold decrease in hydrogen-ion concentration. Therefore, statements that connect low pH with alkalinity are incorrect.
Step-by-Step Solution:Check (a): pH = log10(1/[H+]) → correct.Check (b): higher pH → lower [H+] → correct.Check (c): lower pH → higher [H+] → correct.Check (d): lower pH gives alkalinity → incorrect; lower pH is acidic.
Verification / Alternative check:Example: pH 5 has [H+] 100 times higher than pH 7, confirming acidity increases as pH decreases.
Why Other Options Are Wrong:Only (d) conflicts with the pH definition; the others are standard relationships.
Common Pitfalls:Equating alkalinity (acid-neutralizing capacity) with pH directly; while correlated, alkalinity is a buffering capacity, not simply pH.
Final Answer:Lower pH gives an alkaline solution