Using Freezing-Point Depression to Find Molecular Weight PDF

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Ganim 1 CHM 267, Section 4 Retha Ganim 23 September 2015 Using Freezing-Point Depression to Find Molecular Weight Introduction The main objective in this experiment is to use the freezing point depression equation to determine the molecular weight (MW) of benzoic acid. The freezing point depression can be quantified by the observed effect with the addition of benzoic acid to a solution of lauric acid. The freezing-point depression is a colligative property that depends on the ratio of solute and solvent particles. For instance, when a solute is dissolved in a solvent, the freezing temperature is lowered due to the number of moles of solute added, not necessarily the nature of the substance. Examples of colligative properties are boiling point and the osmotic pressure; they are interested in the amount of the substance used not to indicate the identity of the substance. The equation of freezing-point depression, we will be using in this experiment is Δt= iKf*m. Where Δt is the freezing point depression, Kf is the molal freezing point depression constant (3.9 degrees Celsius*kg/mol), m is the molality of the solution, and i is the van’t Hoff factor, which is the number of particles that are created when a substance is dissolved in a solution. The molality of solution and the van’t Hoff factor are the most important variables in the freezing-point depression. In this experiment, we will determine the freezing temperature of the pure solvent, lauric acid, the freezing temperature of a mixture of lauric acid and benzoic acid, then to calculate the freezing point depression of the mixture and the molecular weight of benzoic acid. This experiment relates to the concept being studied, because we are using the freezing point depression equation to determine the molecular weight of benzoic acid.

Ganim 2 Procedure and Data Sheet Submitted as the carbon-less copy of the pre-lab outline (see attached) and data sheet. Results and Discussion The purpose of this experiment was to use the freezing point depression equation, Δt= iKf*m to solve for the molecular weight of benzoic acid. We noticed the graph of the pure solvent; lauric acid shows a level of horizontal curve as the solid occurs, because a pure substance maintains constant as it freezes. Whereas, the curve for the mixture of lauric acid and benzoic acid solution slopes downward due to when a solution freezes, typically the pure solvent freezes first and as it continues to become more solid, the remaining solution is more concentrated, which the freezing point is lower. We also noticed to prevent super cooling of the freezing point of pure solvent and the freezing point of solution is to stir constantly during the cooling process. Since lauric acid is a molecular solid, the most common intermolecular forces that are present in lauric acid are London dispersion forces. As the molecular liquid freezes, the average kinetic energy of the molecules decrease and slows down, eventually the heat is removed so that the attractive forces can come closer together, so their positions is fixed and the substance freezes. In this experiment, the mass of lauric acid (g) was given at 8 grams and the mass of benzoic acid was given at 1 grams. Our result for freezing temperature of pure lauric acid was 42.5 degrees Celsius and the freezing point of the benzoic acid-lauric acid mixture was 39.1 degrees Celsius. We then calculated the molality (m) by using the formula, Δt= iKf*m, we got 0.87179 mol/kg. Then, to calculate the moles of benzoic acid solute, we multiply the molality with the mass of benzoic acid in kg and we got .00697432 mol. After, we calculated the experimental molecular weight of benzoic acid by using the equation, MW benzoic acid= grams

Ganim 3 benzoic acid*i*Kf/ kg lauric acid * Δt, we got 143.383g/mol. The accepted molecular weight of benzoic acid from its formula, C6H5COOH is 122 grams. Lastly, to calculate the percent discrepancy between the experimental and accepted value is by using the equation, [accepted value- experimental value]/accepted value *100, and we got 17.527%. Any discrepancy identified in this experiment is if the amount of solvent evaporates from the test tube, which the molar mass of the solid would be too small. Sample Calculation Calculating the molecular weight of the benzoic acid Equation: MW benzoic acid= grams benzoic acid * i* Kf divided by kg lauric acid* Δt MW benzoic acid= 1 gram*0.87179 mol/kg*3.9 divided by 0.008kg* 3.4= 143.383 g/mol Accepted value of benzoic acid, C6H5COOH is 122 grams Calculating the percent discrepancy between the experimental and accepted values Equation: [Accepted value- experiential value]/ accepted value *100 [122-143.383]/122*100= 17.527% Conclusion This experiment was conducted to apply the freezing point depression equation, Δt= iKf*m, to determine the molecular weight (MW) of benzoic acid, the freezing temperature of the pure solvent, lauric acid, the freezing temperature of a mixture of lauric acid and benzoic acid, and the freezing point depression of a mixture. Results of the molality (m) is .87179 mol/kg, moles of benzoic acid solute is .00697432 mol, the experimental molecular weight of benzoic acid is 143.383 g/mol, the accepted molecular weight of benzoic acid, C6H5COOH, is 122 grams, and the percent discrepancy between the experimental and accepted values is 17.527%. By using the freezing point depression equation, one can determine the molecular weight of

Ganim 4 benzoic acid, by incorporating the freezing temperature of the pure solvent, lauric acid and the mixture of the solution....