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answer Lle yuesuull decreases? Please use plot (a) to (e) Pure benzene freezes at 5.4 °C and a solution of 0.223 g of phenylacetio acid (C&H;,CH2COOH) in 4.4 g of benzene freezes at 4.44 °C. The molar freezing point lowering constant of benzene is 5.12 K/mol, please comment on the result and what kind of intermolecular force exists in this compound? (5%) 4.1 g of PolyN-isopropylacrylamide) (PNIPAm, water soluble polymer) is dissolved in 100 g of water at room temperature in a sealed glass vial and observed to be perfectly clear solution. The vial is then taken to the laboratory sink and placed under a stream hot water. The polymer solvent mixture now becomes turbid and looks like milk. Next the vial is placed under a stream of water from the cold water tap and the polymer mixture reverts to a perfectly clear solution. How would you explain this phenomenon? (10%) When 1 mole of water supercooled to-10 freezes isothermally, what are the Ventropy change of the system and surroundings? Give the molar enthalpy of the melting of ice at 0 °C is 6025 J/mol, the molar heat capacities of ice and water are

Explanation / Answer

e)

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

dTb = Kb*molality * i

where:

Kf = freezing point constant for the SOLVENT; Kb = boiling point constant for the SOLVENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

Tb mix = Tb solvent - dTb

Tbenzne = 5.4°C

m = 0.223 g of phenylacetic acid

m = 4.4 g benzene = 4.4 *10^-3 kg

dTf = Kf*m

(4.44-5.4) = -5.12 * mol of solute / (4.4 *10^-3)

mol of solute= (4.44-5.4)/-5.12 *  (4.4 *10^-3) = 0.000825

mass of acid = mol*MW = 0.000825*136.15 = 0.1123 g

note that

m = 0.223 g was added

therefore,

ratio = 0.223 /0.1123 = 1.98

this implies there are 2 ions in solution

note that

phenyl acetic acid will form H+ protons and the phenylacetate ion, therefore we must account this in "i"

i = 2 for this species

the forces must be:

ion-dipole interactions mostly

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