Discussion
Lake water at pH \( 4.5 \) is compared with unpolluted water at pH \( 6.5 \). The hydrogen ion concentration is greater by a factor of
Answer: C
Correct answer: 100
pH is the negative logarithm of hydrogen ion concentration, so a difference of two pH units is a factor of \( 10^2 \):
\[ \frac{[H^+]_{4.5}}{[H^+]_{6.5}} = 10^{6.5-4.5} = 10^2 = 100 \]
The distractor \( 2 \) treats the scale as linear, which is the commonest error and badly understates the change. At pH 4.5 most fish species cannot survive, both from the acidity itself and from the aluminium that low pH mobilises out of catchment soils.
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