K_w (ionic product of water at 25 °C) is:
A1 × 10⁻¹⁴
B1 × 10⁻⁷
C55.5
D1 × 10⁷
Answer & Solution
Correct answer: A. 1 × 10⁻¹⁴
1. Pure water self-ionises: 2 H₂O ⇌ H₃O⁺ + OH⁻.
2. At 25 °C: [H⁺][OH⁻] = K_w = 10⁻¹⁴.
3. So [H⁺] = [OH⁻] = 10⁻⁷ M and pH = 7 (neutral).
4. K_w increases with temperature (self-ionisation is endothermic).
_Source: NCERT Class 11 Chem Ch 7 §7.10 Ionisation of Water_
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