Question

What are the major species present and equilibrium expressions at the half equivalence point, equivalence point, and 3/2 equivalence point in titrations with: 0.10 M NaOH and a) 0.1M HCl b) 0.1M CH3COOH c) 0.05M H3PO4 d) hl-mandelic

Answer 1

Equivalence point is that condition in a titration when the amount of titrant added is just sufficient to completely neutralize the analyte.

a) 0.1M NaOH and 0.1M Hcl

At half equivalence point, 2NaOH + Hcl -------------------> Na⁺ + Nacl + H₂O + OH^- The major species present are hydroxide ions, sodium chloride salt, sodium ions, water molecules.

At equivalence point, NaOH + Hcl --------------> Nacl + H₂O The major species present are sodium chloride salt and water molecules.

At 3/2 equivalence point, 2NaOH + 3Hcl ----------------> 2Nacl + 2H₂O + H⁺ + cl^- The major species present are sodium chloride, water molecules, protons and chloride anions.

b > 0.1M CH₃COOH and 0.1M NaOH   

At half equivalence point, 2NaOH + CH₃COOH ------------> CH₃COONa + Na⁺ + H₂O + OH^- The major species present are sodium acetate salt, sodium ions, hydroxide ions and water.

At equivalence point, NaOH + CH₃COOH ------------> CH₃COONa + H₂O The major spcies present are sodium acetate and water molecules.

At 3/2 equivalence point, 2NaOH + 3CH₃COOH --------------> 2CH₃COONa + CH₃COO^- + 2H₂O + H⁺ The major species pressent are sodium acetate salt, acetate anions, water molecules and protons.

c > 0.05M H₃PO₄ and 0.5M NaOH

Now as phoshoric acid is a tribasic acid so one mole of phosphoric acid neutralises three moles of sodium hydroxide.  

At half equivalence point, H₃PO₄ + 6 NaOH ---------------> Na₃PO₄ + 3Na⁺ + 3OH^- + 3H₂O At half equivalence point condition only half of the analyte or half of the moles of sodium hydroxide gets neutralised. So here,of the six volumes of sodium hydroxide three volumes are neutralised. The major species present are trisodium phosphate salt, sodium cations, hydroxide ions, water molecules.

At equivalence point, H₃PO₄ + 3NaOH ---------------> Na₃PO₄ +3H₂O The major species present are trisodium phosphate salt and water molecules.

At 3/2 equivalence point, 3H₃PO₄ + 6NaOH -------------> 2Na₃PO₄ + 3H⁺ + 3PO₄^3- + 6H₂O The major species present are trisodium phosphate salt, protons, phosphate anions, water molecules.

d > 0.5M NaOH and hl-mandelic

At half equivalence point, 2NaOH + hl-mandelic acid --------------> sodium mandalate + Na⁺ + H₂O + OH^-   The major species present are sodium mandalate salt, sodium ions, water molecules and hydroxide ions.

At equivalence point, NaOH + hl-mandelic acid ---------------> sodium mandalate + H₂O The major species present are sodium mandalate salt and water molecules.

At 3/2 equivalence point, 2NaOH +3hl-mandelic acid -----------> 2sodium mandalate + 2H₂O + H⁺ + mandalate^- The major species present are sodium mandalate salt, water molecules, protons and mandalate anion.