4. Smart-asa Gnome JusTy craves the smell of moth balls (it?s why his socks neve

Question

4. Smart-asa Gnome JusTy craves the smell of moth balls (it?s why his socks never ever have holes). He obtains the cooling curve for pure paradichlorbenzene (PDB) and determines its freezing point to be 72.0?C. He then dissolves 1.30 g of naphthalene (C10H8) in 30.25 g of PDB and finds the solution freezing point to be 67.5?C. a) Find the freezing point depression constant (Kf) for PDB: b) JusTy next dissolves 2.35 g of Benzil in 30.46 g of PDB and finds the freezing point to be 68.4?C. Calculate the molar mass of Benzil.

Explanation / Answer

a) First we use the next formula: tfa - tfsol = Kf x gste / MMste x kgsnt ----> Kf = (tfa - tfb) x MMste x kgste / gste

Molar mass of naphthalene: (12x10) + 8x1 = 128 g/mol

Kf = (72 - 67.5) x 128 x 0.03025 / 1.3 ----> Kf = 13.40 kg

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