Using Freezing-Point Depression to Find Molecular Weight

 

DATA TABLE

 

 

 

DATA ANALYSIS

 

 

1. Calculate molality (m), in mol/kg, using the formula DT = Kf ´ m. The Kf value for lauric acid is 3.9°C•kg/mol.

 

 

2. Calculate moles of benzoic acid solute, using the molality and the mass (in kg) of lauric acid solvent.

 

 

 3. Calculate the experimental molecular weight (molar mass) of benzoic acid, in g/mol.

 

 

4. Determine the accepted molecular weight (molar mass) of benzoic acid from its formula, C6H5COOH.

 

 

5. Calculate the percent discrepancy between the experimental and accepted values. Using Freezing Point Depression to Find Molecular Weight QUESTIONS

 

 

6. What is the equation for calculating molality? Why do we use molality rather than molarity as our concentration unit for this experiment?

 

 

7. Use the equation above along with the freezing-point depression equation to derive an expression for calculating the molar mass of a solute. (Hint: Consider the types of calculations you used in questions 1-3 and try to put them together into one mathematical formula.)

 

 

8. What types of intermolecular forces are present in a molecular solid such as lauric acid? Describe what is happening with regard to intermolecular forces as a molecular liquid freezes.

 

 

9. A student spills some of the solvent before the solute was added. What effect does this error have on the calculated molar mass of the solute? Mathematically justify your answer.

 

 

10. A different student spills some of the solution before the freezing-point was determined. What effect does this error have on the calculated molar mass of the solution? Justify your answer.

 

 

11. The freezing point of diet soda is higher than the freezing point of regular soda, but lower than 0 °C, the freezing point of pure water.

 

a) Explain why both diet and regular soda freeze at temperatures lower than 0 °C.

 

b) Why does diet soda freeze at a higher temperature that regular soda?

 

 

12. In order to find the molar mass of an unknown compound, a research scientist prepared a solution of 0.930 g of unknown in 125 g of a solvent. The pure solvent had a freezing point of 74.2 °C, and the solution had a freezing point of 73.4 °C. Given the solvent’s freezingpoint depression constant, K f = 5.50 °C/m, find the molar mass of the unknown.

 

 

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