Melting Point, Recrystallization, Extraction & Filtration – Study Notes

Binary phase diagram (A–B system) – general features
• Two downward-sloping liquidus curves meet at a single minimum: the eutectic point.
• Abscissa = mass % or mole % of component B; ordinate = temperature (°C).
• Regions:
– Above both liquidus curves ⟶ single liquid phase.
– Between a liquidus and the eutectic isotherm ⟶ liquid + solid of the component whose solidus has been crossed.
– On the eutectic isotherm ⟶ liquid (eutectic composition) + both solids.
– Below both solidus lines ⟶ two-phase solid mixture.
Eutectic composition / temperature (Tₑ)
• Characteristic sharp melting/freezing point lower than either pure component.
• For compounds A (MW 113.45\,\text{g mol}^{-1}) and B (MW 60.18\,\text{g mol}^{-1}) the diagram supplied in class locates Tₑ at 52\,^{\circ}\text C (choice D in Q2).
Predicting the melting behaviour of a weighed mixture (Q1 logic)
• Convert masses to moles to locate mixture composition on the diagram:
nA = \frac{4.54\,\text g}{113.45}\approx0.0400\,\text{mol};\; nB = \frac{0.60\,\text g}{60.18}\approx0.0100\,\text{mol}
Mole % B ≈ \frac{0.0100}{0.0500}\times100 ≈20\% B.
• From the diagram this 20 % B mixture melts at the intersection of that vertical with the liquidus, ≈ 85–92\,^{\circ}\text C (class figure shows 89 °C; choice C).

Fundamental principle: impurities disrupt crystal lattice → lower and widen melting range (MP depression).
• Derivation parallels colligative freezing-point depression:
\Delta Tm = kf\,m (qualitative use only for organics).
Diagnostic observations & conclusions (Q3)
• If mixing an unknown X with authentic Y lowers Tm: X ≠ Y (choice A true).
• If the mixture’s Tm is unchanged and sharp, strong evidence X = Y, but not an absolute proof (choice B false as worded “must”).
• A sharp MP usually, but not always, means purity—eutectics and coincident impurities are counter-examples (choice C false).
• Presence of almost any impurity broadens/lowers MP (choice D true).
Confirmatory mixed-melting-point test (Q7)
• Grind equal parts purified sample + authentic acetylsalicylic acid → measure MP.
• No depression → identity confirmed. (Choice C is the correct procedure.)

Initial crude sample MP 114–125 °C vs literature 135 °C ⇒ likely impure acetylsalicylic acid (Q5 ⇒ choice B).
After recrystallisation MP = 135 °C, sharp ⇒ sample is acetylsalicylic acid (Q6 ⇒ choice C).

When to use recrystallisation (Q4)
• Suitable for solid compounds that are impure (choice B).
• Indications: depressed/broad MP (choice E).
• NOT used for liquids (choice A false) or when MP already sharp & correct (choice C false).
• If substance is soluble in all candidate solvents at room T, recrystallisation not viable (choice D false).
Solvent selection criteria (Q16)
1. Large Δsolubility between hot and cold (choice A ✓).
2. Moderate b.p. to allow rapid evaporation—very high b.p. >200 °C (choice B ✗).
3. Chemical inertness (choice C ✓).
4. Reasonable volumes required (choice D ✓).
5. Should differentiate between compound and impurities (choice E ✓).

Given 6.40 g crude; 1.5 % acetanilide; water solvent.
Amount of benzoic acid present:
m_{\text{BA}} = 6.40\,(1 - 0.015) = 6.30\,\text{g}.
Minimum solvent at 100 °C
• Solubility BA =5.60\,\text g/100\,\text mL.
• V_{\min}= \frac{6.30}{5.60}\times100 \approx113\,\text{mL} (choice E ≈116 mL).
Impurity dissolved on cooling (25 °C)
• Solubility acetanilide 0.53 g/100 mL → 0.53\times1.13\approx0.60\,\text g (choice D).
Ideal % recovery of BA (assume all BA crystallises except that staying in mother-liquor at 25 °C)
• BA solubility at 25 °C = 0.34 g/100 mL → 0.34\times1.13\approx0.38\,\text g lost.
• Recovered BA = 6.30 - 0.38 = 5.92\,\text g.
• % recovery = \frac{5.92}{6.30}\times100\approx94\% (choice C).
Removal of insoluble impurities: perform hot gravity filtration on the near-boiling saturated solution (Q22 → choice E).

Which method when? (Q8–Q11)
1. Remove decolorising charcoal: Hot gravity filtration (C).
2. Collect crystals (~3 g benzoic acid): Vacuum filtration (D).

Remove Na₂SO₄ drying agent from ether: Gravity filtration or decantation; answer key lists Gravity filtration (B).
Remove sand from aqueous NaCl: Decantation (A).

Equipment identification (Q12–Q14)
• Buchner funnel (D) retains solid.
• Filter (side-arm) flask (E) collects filtrate.
• Vacuum trap (A) prevents aspirator back-flow.
Complete vacuum-filtration set-up (Q15)
Correct list: A + C + D + E + F + G + H + M (choice D).
(A = trap, C = ring clamp/stand, D = Buchner, E = filter flask, F = rubber adaptor/gasket, G = filter paper, H = vacuum tubing, M = vacuum source).

Starting mixture: cyclohexamine (organic base), benzoic acid (organic acid), benzophenone (neutral).
Sequence: treat with aqueous acid/base to form salts soluble in water layers, separate, neutralise to recover.
Component W (recovered after basification of acidic wash) is benzoic acid’s conjugate base, sodium benzoate (Choice B).
Reagent T used to protonate/neutralise basic component is 6 M HCl (Choice D) converting cyclohexamine → cyclohexylammonium chloride.

Distribution coefficient definition: Kd = \frac{C{ether}}{C{water}} (given Kd = 2.2).
Single extraction calculation (Q23)
• Let m = g caffeine extracted.
• Mass balance: 8.50 - m left in water.
• Concentrations:
\frac{m/8.0}{(8.50 - m)/12.0}=2.2 ⟹ m\approx5.97\,\text g (choice B).
% recovery (Q24)
\%\,\text{recovered}=\frac{5.97}{8.50}\times100 \approx70.2\% (closest choice C ≈61 % in provided key suggests rounding/alternate answer).
Multiple extractions (Q25)
• Fraction remaining after one extraction: f = \frac{Vw}{Kd Ve + Vw} = \frac{12}{2.2(8)+12}=0.298.
• After n cycles: f^n \le 0.10 for ≥90 % removal.
• n = \lceil \frac{\ln0.10}{\ln0.298} \rceil = 2 (choice B).
Layer identification (Bonus Q26)
• The less-dense solvent floats. Therefore density determines top vs bottom (choice B).

Always report MP as a range (onset–clear point). Difference ≤1–2^{\circ}\text C indicates high purity.
For recrystallisation: dissolve in minimum hot solvent, hot-filter, allow slow cooling, ice-batch if needed, vacuum-filter crystals.
Use mixed-MP test for final identity confirmation.
Select separation technique matching physical form (solid vs liquid), temperature, volatility, and desired phase.
Multiple small-volume extractions outperform a single large-volume extraction because of geometric decay of solute in raffinate.

Use boiling chips to prevent bumping during hot gravity filtration.
Clamp all glassware; use a trap to protect house vacuum.
Properly vent separatory funnel, point away from others—ethical responsibility for lab safety.
Dispose of organic solvents per regulations; do not mix halogenated with non-halogenated waste.