Concept:Dilution affects both the volume of the solution and the degree of ionization (Ostwald's Dilution Law), but the overwhelming effect of the added volume dominates the absolute concentration metric (Molarity).
Formula:$$ C_1V_1 = C_2V_2 \quad \text{and} \quad \alpha = \sqrt{\frac{K_a}{C}} $$
Solution:- Adding water dramatically increases the total volume (\( V \)) of the solution.
- According to Ostwald's law, dilution does cause a weak acid to ionize more (\( \alpha \) increases), producing slightly more total moles of \( \text{H}^+ \).
- However, the massive increase in the denominator (Volume) far outpaces the small increase in the numerator (moles of \( \text{H}^+ \)).
- Therefore, the overall molar concentration (moles/Liter) of \( \text{H}^+ \) ions must definitively decrease.
Why other options are incorrect:While total moles of \( \text{H}^+ \) might increase, the
concentration (which is what pH and this question measure) always drops upon dilution.
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