Dissolution of a solute in a solvent, results in lowering of the freezing point

Question

Dissolution of a solute in a solvent, results in lowering of the freezing point at the solvent. The phenomenon known as freezing point depression (FPD) is a colligative property it is dependent of the amount of volute present, and not the nature of the solute. The relationship between FPD and amount of solute is mathematically represented as deltaT = k_t, m where delta T = freezing point depression. K_a = freezing point constant for the solvent. M = molarity of the solution i.e. the mol of solute/kg of solvent The relationship can be used to determine the molecular weight of the solute. Use the graph paper provided and the experimental data presented below, to determine a) the molecular weight of the benzoic acid (solute) added to auric acid (solvent) and b) percentage error. The freezing point constant for caustic acid is 3.9 -C kg/mol. Show all calculations

Explanation / Answer

From the given data,

depression is freezing point (dT) = 38 - 35 = 3 oC

molality of lauric acid-benzoic acid solution (m) = 3/3.9 = 0.77 m

moles of benzoic acid = 0.77 x 0.008020 = 0.0062 mols

molar mass of benzoic acid = 1.040/0.0062 = 167.74 g/mol

Percent error = (167.74 - 122.12) x 100/122.12 = 37.36%

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