It was 11:47 PM. Her desk was a disaster of coffee rings, annotated periodic tables, and the carcass of a Bic pen she’d chewed to death. Question 9 of the 9-pack stared up at her. A 7-marker on calculating the pH of a weak acid-strong base titration at the equivalence point —but with a twist: a diprotic acid. Sulfurous. H₂SO₃. Stepwise Ka values. A salt hydrolysis that seemed designed by a sadist.
It wasn’t a ghost. It was a mark. The decimal point was a cold spike in her chest, the zero a mocking mouth. Her first HSC Chemistry assessment task back. She had cracked —not the exam, but herself. Two hours of staring at equilibrium constants until they swam off the page like startled fish.
And somewhere inside, where the 9.04 used to live, she found a solid 92.
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. hsc chemistry 9 crack
Her father had knocked gently. "Mira? Everything okay?"
She had not avoided the cracks. She had crawled inside them, felt the rough edges, and found that the light still got through.
Step one: The weak acid. H₂SO₃. It gives up one proton. Becomes HSO₃⁻. Ka1. Like the first domino. It was 11:47 PM
She wrote her answer in full sentences. Explained the hydrolysis. Compared Ka2 and Kb. Showed the approximation. Concluded pH = 4.40. Then she put her pen down.
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."
She flipped to the data sheet. Ka1 of H₂SO₃ = 1.54 × 10⁻². Ka2 = 1.02 × 10⁻⁷. Kb for HSO₃⁻ = Kw/Ka1 = (1×10⁻¹⁴)/(1.54×10⁻²) = 6.49×10⁻¹³. A 7-marker on calculating the pH of a
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.
Mira put her head on the desk. The wood was cool. She could smell highlighter ink and her own exhausted sweat.
Her eyes snapped open. She grabbed a fresh page.