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Colligative Properties Lab...

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Colligative Properties Introduction There are four colligative properties; vapor pressure, boiling point, freezing point and osmotic pressure. When a nonvolatile solute is added to a solvent, it effects the colligative properties. The vapor pressure of the solution is lower than the vapor pressure of the pure solvent. This means that a larger amount of energy needs to be applied to the solution to make to vapor pressure equal the atmospheric pressure. The solution will then have a higher boiling point than the original solvent. This concept also applies to the freezing points of the solution. A solution will have a lower freezing point than the pure solvent. For both processes, the solute particles cause the changing in colligative properties. In a solution, the number of particles being added depends on the number of ions in which the ionic compound will dissociate into. The compound that dissociates into the most amount of particle will affect the colligative property the most, a strong electrolyte will cause the same affect. The following equations are used 1) and 2). The constants in this equation are kb and kf. Molality will be used for the concentration and the i represents the Van’t Hoff factor. 1) 2)

∆ Tf =ikfm ∆ Tb=ikbm

The purpose of this lab was to figure out the constant of water using an aqueous solution of sodium chloride and then finding the molar mass of ethylene glycol and the unknow substance. This affect is witnessed in “real life scenarios” during wintertime when salt is spread on icy roads, causing the freezing point of water to be lowered. Results and Discussion The constant of kb for water was found to be 0.59. It is known that the Van’t Hoff factor is 2, with this to molality for the mass of NaCl. This can then be converted to moles and the mass in grams of water used can be converted into kilograms. The ∆ Tb was found by getting the difference between the boiling points of water, found to be 100.3, and NaCl, found to be 104.4. To calculate the molar mass of ethylene glycol the same equation was used, changing slightly to find molality and then convert to molar mass of ethylene glycol. The kb value used was 0.59 and the Van’t Hoff factor used was 1. For the ∆ Tb it was calculated by taking the boiling point of ethylene glycol, found to be 104.6, and subtracting the boiling point of water, which was found to be 100.3. The molality, moles over kg or solvent, was found to be 8.58. The mass of solvent in grams was converted to kilograms and found to be 0.099 kg. The molar mass was found to be 62.08 g/mol. For the unknown substance, the molar mass was also to be found. The molar mass of the substance was 40.4 g/mol. The error for the molar mass of ethylene glycol for this experiment was found to be 0.02%. This is an acceptable rate of error for this experiment. No large sources of error were found. Conclusion In this experiment it was asked that the boiling point of water, and the molar mass for ethylene glycol and the unknow substance be found. It was seen that a nonvolatile solute can affect the colligative properties of the solvent, shown by using the equation ∆ Tb=ikbm ....