Acids, Bases, Salts & Acid-Base Titration
Properties of acids and bases, Arrhenius and Brønsted-Lowry definitions, pH scale, chemical indicators, neutralization reactions, salt preparation methods, and acid-base volumetric titration.
In Ghanaian agriculture, acidic soils in high-rainfall forest zones (Western and Eastern Regions) are limed with powdered limestone (calcium carbonate, CaCO₃) or slaked lime (Ca(OH)₂) to raise soil pH to optimal levels for cocoa and cassava cultivation.
Key Syllabus Learning Objectives
- 1Differentiate between strong and weak acids/bases and define pH mathematically and conceptually
- 2Compare indicators (litmus, phenolphthalein, methyl orange) and choose appropriate indicators for specific titrations
- 3Outline four distinct chemical methods for preparing soluble and insoluble salts with balanced equations
- 4Perform acid-base titration calculations using C_a V_a / C_b V_b = n_a / n_b
Volumetric Analysis: Acid-Base Titration Technique
- Apparatus: Pipette delivers fixed volume of base into conical flask; Burette dispenses variable volume of acid with stopcock control.
- Titre Concordancy: Titrations are repeated until at least two or three titre values agree within ±0.10 cm³ (concordant titres).
- Indicator Choice: Strong Acid + Strong Base = any indicator; Strong Acid + Weak Base = Methyl orange; Weak Acid + Strong Base = Phenolphthalein.
Quality control chemists at pharmaceutical plants in Accra titrate antacid tablets against standardized HCl to verify active magnesium hydroxide content.
Concordant titres must fall within 0.10 cm³ of each other; the initial reading must be subtracted from the final burette reading.
Classification & Solubility of Salts
- All nitrates (NO₃⁻) and all ethanoates are completely soluble in water.
- All sodium, potassium, and ammonium salts are soluble.
- Most chlorides are soluble EXCEPT Lead(II) chloride (soluble in hot water) and Silver chloride (AgCl).
- Most sulfates are soluble EXCEPT Barium sulfate (BaSO₄), Lead(II) sulfate (PbSO₄), and Calcium sulfate (slightly soluble).
- All carbonates are INSOLUBLE EXCEPT Group 1 carbonates (Na₂CO₃, K₂CO₃) and (NH₄)₂CO₃.
The white precipitate produced when testing for sulfate ions using barium chloride solution confirms whether well water contains dissolved sulfate minerals.
Insoluble salts like BaSO₄ and AgCl are prepared by precipitation (mixing two soluble aqueous salt solutions).
- In titration tables, always record burette readings to two decimal places (e.g. 24.50 cm³, not 24.5 cm³).
- Average titre must only be calculated from concordant titres (values within 0.10 cm³ of each other).
- State the formula: C_a V_a / C_b V_b = n_a / n_b clearly before substituting values to secure method marks.
- Using dirty burettes or blowing out the last drop of liquid retained in the tip of a pipette (pipettes are calibrated to retain that drop).
- Confusing acid strength (degree of ionization) with acid concentration (amount of solute dissolved per dm³).
- Choosing phenolphthalein for a strong acid-weak base titration (endpoint occurs in the acidic range, requiring methyl orange).
- Can I calculate pH given hydrogen ion concentration [H⁺]?
- Can I write balanced equations for the 4 methods of salt preparation?
- Can I calculate the molar concentration of an unknown acid from titration data?
- Can I list which chlorides, sulfates, and carbonates are insoluble in water?
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