A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 17 of 21Electrolytes in solution
Électrolytes en solution · الإلكتروليتات في المحاليل
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Some substances split completely into ions in water, others only partly. This decides how acidic a solution is and how well it conducts.
Key ideas
- Strong electrolytes ionise completely: strong acids, strong bases, most salts.
- Weak electrolytes ionise partly: CH₃COOH, NH₃·H₂O, water; this is an equilibrium.
- Water: Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25 °C.
- Hydrolysis of salts: CH₃COONa is basic, NH₄Cl is acidic, NaCl is neutral.
Formulas to know
- HCl → H⁺ + Cl⁻ (complete)
- CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partial)
- pH + pOH = 14 at 25 °C
Worked example
Find the pH of 0.01 mol/L NaOH at 25 °C.
- Strong base: [OH⁻] = 0.01 = 10⁻² mol/L
- Kw: [H⁺] = 10⁻¹⁴ / 10⁻² = 10⁻¹² mol/L
- pH: −log(10⁻¹²) = 12
Answer: pH = 12
Practice questions
1. Which is a weak electrolyte?
NaCl · HCl · CH₃COOH ✓ · NaOH
Acetic acid ionises only partly.
2. pH of 0.001 mol/L HCl?
1 · 3 ✓ · 11 · 0.001
[H⁺] = 10⁻³.
3. A solution of NH₄Cl in water is:
acidic ✓ · basic · neutral · it depends on the volume
NH₄⁺ hydrolyses and releases H⁺.