There are many chemical properties that have been utilized by humans in order to suit our own needs. One of these properties that we have many uses for is a property known as colligative properties. We have used colligative properties to manipulate many things and it is because of these properties that we are able to lower the temperature of a solution so that we can make ice cream. In this lab, you will be studying how a basic property of solutions allows for the lowering of a freezing point and the raising of a boiling point.
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Colligative properties are those properties of solutions that depend on the number of dissolved particles in solution, but not on the identities of the solute. This means that these properties depend only on the amount of solute you have and NOT the identity of the solute that you are adding. The two properties that are most often effected by the identity of the solute are the (1) boiling point and (2) freezing point.
When you add solute to a solution you both raise the temperature at which it boils and lower the temperature at which the solution freezes. The freezing point is lowered because you have added more solute molecules to the solution. More solute molecules means that water molecules have a harder time locating one another, therefore, forming the solid arrangement becomes harder because of the solutes blocking the formation of this structure. This results in a lower freezing point for the solution when you add solute molecules. The boiling point of a solution is increased when you add solute molecules due to the increased amount of solute molecules. When you have more molecules in solution, then you have more molecules blocking the ability of the water molecules from escaping the solution and boiling. This means that a higher temperature is required in order to move water molecules from the liquid phase to the gas phase (i.e. boiling). |
Determining Molar Mass Using Colligative Properties:
We can calculate the molar mass of a solute from the measurement of freezing point depression and boiling point elevation. If you can measure the amount of substance initially and the change that it causes in the boiling point, then you will be able to calculate the molality because this will be the only variable remaining. Recall that molality is calculated by: Therefore, if you know the change in boiling point or freezing point, then you can solve for the molality of the solution. Once molality is known, then you can determine the moles of solute that you have:
Since you know the amount of solvent that you used, you can solve for the amount of moles of solute. If you then know how much solute you started with, then you can determine the molar mass through the following equations.
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