That was three weeks ago. Now, the real HSC was six days away, and Mira had a new kind of crack in her hands: a set of nine past paper questions, printed out, stapled messily in the corner. Chemistry 9-Pack: Hardest Questions from 2019–2024. Her tutor had given it to her. "These are the ones that separate the Band 6 from the rest," he’d said. "Crack these, and you crack the code."
Compare Ka2 (1.02×10⁻⁷) to Kb (6.49×10⁻¹³). Ka2 is much larger . So the HSO₃⁻ acts as a weak acid. The solution is slightly acidic. Of course. The pH at equivalence is below 7. Not neutral. That was the trap. hsc chemistry 9 crack
When she walked into the exam hall six days later, she saw a 7-marker on weak acid-strong base titration. Diprotic. Not sulfurous—carbonic this time. But the bones were the same. That was three weeks ago
Step two: Add NaOH. The strong base. They neutralise. But at equivalence? No excess base. Only the conjugate base remains. HSO₃⁻. But wait—HSO₃⁻ is amphiprotic . It can act as an acid or a base. She had forgotten that the first time she tried this question. Her tutor had given it to her
She wrote: At equivalence point for first proton: species present = HSO₃⁻. This hydrolyses in water. Two equilibria: HSO₃⁻ + H₂O ⇌ H₂SO₃ + OH⁻ (Kb1) AND HSO₃⁻ ⇌ H⁺ + SO₃²⁻ (Ka2). Since Ka2 > Kb1, solution is acidic? No—check values.
Step one: The weak acid. H₂SO₃. It gives up one proton. Becomes HSO₃⁻. Ka1. Like the first domino.