ARCH-C-07: Solubility Curves (g per 100 g Water), Recrystallisation Mass & Paper/TLC Chromatography R_f

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-C-07: Why the 145s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~145s (Blows 90s Clock)

Per

100\text{ g}

of water, cooling from

60\,^\circ\text{C}

20\,^\circ\text{C}

precipitates

110 - 30 = 80\text{ g}

\text{KNO}_3

. Scaling to

50\text{ g}

of water gives

\frac{50}{100} \times 80 = 40\text{ g}

(~100s).

The Fast ShortcutTarget: 40s (Saves 105s)

**Line 1 (

\Delta S

per

100\text{ g}

):**

\Delta S = 110 - 30 = 80\text{ g}

.
**Line 2 (Scale to

50\text{ g}

Water):**

\frac{50}{100} \times 80 = 40\text{ g}

(20s).

Where Students Lose Time: In 10 past UAT-UK questions (4.44% of Chemistry), candidates lose 105s by if solubility at t_hot is 80 g / 100 g water, 100 g of saturated *solution* contains 100*(100/180) g of water, not 100 g of water! Instead, apply C8 with Maths 1 skill M3.5.

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