Use trial one. 2.407 g cyclohexane solvent (Q.3) Original mass biphenyl-0.07g (Q

Question

Use trial one. 2.407 g cyclohexane solvent (Q.3) Original mass biphenyl-0.07g (Q.4) Unsure if my calculations are correct, large percent error. PROCESSING THE DATA 1. Determine the difference in freezing temperatures, At, between the pure cyclohexane (ti) and the mixture of cyclohexane and biphenyl (t2). Use the formula, At t1 t2 Calculate molality (m), in mol/kg, using the formula,At = Kf. m (Kf= 20.2°C-kg/mol for cyclohexane). 2. 3. Calculate moles of biphenyl solute, using the answer in Step 2 (in mol/kg) and the mass (in kg) of cyclohexane solvent. Calculate the experimental molar mass of biphenyl, in g/mol. Use the original mass of biphenyl from your data table, and the moles of biphenyl you found in the previous step. 4. 5. Compare your experimentally determined molar mass of biphenyl with the known value. 6. Calculate the percent error.

Explanation / Answer

Determination of molar mass by freezing point depresssion

Kf = 20.2 oC/m

mass biphenyl = 0.07 g

mass cyclohexane = 2.407 g = 0.002407 kg

molality = moles of solute/kg of solvent

1. dTf = 4.4 oC and 4.5 oC from two trials

Average dTf = 4.45 oC

2. molality of solution = dTf/Kf

                                   = 4.45/20.2

                                   = 0.2203 m

3. moles biphenyl = 0.2203 m x 0.002407 kg

                             = 5.3 x 10^-4 mol

4. molar mass (experimental) of biphenyl = 0.07 g/5.3 x 10^-4 mol

                                                                  = 132.075 g/mol

5. Literature molar mass biphenyl = 154.21 g/mol

We see a large difference between experimental and literature known values.

6. percent error = (154.21 - 132.075) x 100/154.21 = 14.35%

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