can someone help me please Data Sheet 3 Il. Determining the Molar Mass of an Unk

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can someone help me please

Data Sheet 3 Il. Determining the Molar Mass of an Unknown unknown identification code mass of test tube, g mass of test tube plus naphthalene, g mass of test tube plus naphthalene plus unknown, g mass of naphthalene,g mass of unknown, g time, min temperature, "C time, min emperature, "C 2. 12 So 15 ob temperature at which crystals first appeared, 'C maximum temperature attained after crystals appeared, 'C freezing point of naphthalene/unknown solution, "C freezing point depression, "C (At) molality of naphthalene/unknown solution, mol/kg im) experimental molar mass of unknown, g/mol actual molar mass of unknown, g/mol percent error in the determination, % obution 15

Explanation / Answer

And. #I. Freezing point of solution (tsolution) = 74.00C

The freezing point of a solution shall be taken as the temperature at which crystals appear first. Note that the temperature remains constant at freezing point till all the solution is frozen/ crystallized.

#II. Freezing point of pure solvent (naphthalene) = 80.260C

Freezing point depression (dTf) = Freezing point of (pure solvent – solution)

                                                Or, dTf = 80.260C – 74.00C

                                                Hence, dTf = 6.260C

#III. Depression in freezing point of the solution is given by-

                        dTf = i Kf m             - equation 1

            where, i = Van’t Hoff factor.           [ let i = 1 ]

                        Kf = molal freezing point depression constant of solvent = 6.940C / m

                        m = molality of the solution

                        dTf = Freezing point of pure solvent – Freezing point of solution

Putting the values in equation 1-

            6.260C = 1 x (6.940C / m) x m

            Or, m = 6.260C / (6.940C / m)

            Hence, m = 0.902 m

Therefore, molality of solution = 0.902 m

#IV. Let the molar mass of unknown be X g/mol

Moles of unknown sample = Mass / Molar mass = 1.0 g/ (X g/mol) = (1/X) mol

Mass of pure solvent naphthalene = 5.0 g = 0.005 kg

Molality of solution = Moles of unknown solute / Mass of pure solvent in kg

                                    = (1/X) mol / 0.005 kg

                                    = (200/X) mol/ kg

                                    = (200/X) m

# The calculated molality of solution 0.902 m must be equal to (200/X) m.

Or,

            (200/X) m = 0.902 m

            Or, X = 200/ 0.902

            Hence, X = 221.73

Therefore, molar mass of unknown sample = X g/mol = 221.73 g/ mol

# Note: Actual molar mass must be provided or can be deduced after identification of the unknown. % error in determination can only be determined after the actual molar is known.

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