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.
The number 9.04 haunted Mira.
The crack in the textbook spine. The crack in her confidence. The crack in Question 9.
"Fine," she lied, picking up the textbook. The spine was now cracked. A thin white line, like a fault in rock. hsc chemistry 9 crack
She calculated pH using the approximation for an amphiprotic: pH = (pKa1 + pKa2)/2. pKa1 = 1.81. pKa2 = 6.99. Average = 4.40.
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
A clean number. 4.40.