help with postlab I always do terrible Section W(3pm) Th(9am) Th(1pm) Adding sal
ID: 1083577 • Letter: H
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help with postlab I always do terrible
Section W(3pm) Th(9am) Th(1pm) Adding salt to an ice-water bath does lower the freezing point of water. But why does the bath get colder? Explain 1. 2. What effect would each of the following have on the calculated molar mass of an unknown, as determined using the Procedure described in this experiment. Please note if the molar mass would be unchanged, increased, or decreased and explain. a. The temperature probe consistently read 0.15 C higher than it should have over the entire experimental temperature range. The masses of the unknown and the water were determined, and the unknown was completely dissolved in the water. As the test tube was being placed in the ice/salt water bath, your lab partner spilled a small portion of the solution. b. The unknown contained a small amount of insoluble impurity. c. **Don't miss the back side!* lar Mass Determination by Freezing Point Depression).docxExplanation / Answer
1) Melting point is a property of solids. The melting point is simply the temperature at which the solid turns into a liquid. For example, the melting point of normal ice is 0°C at standard pressure.
Freezing point is a property of liquids. The freezing point is simply the temperature at which the liquid turns into a solid. Thus the freezing point of normal freshwater is 0°C.
The value of the melting and freezing points is the same (ie. both are 0°C for water)
When salt is added to water it lowers the freezing point. In other words the water needs to be chilled to a temperature lower than 0°C (say -2°C for example) in order to change to ice. Another way to say the same thing is that salt allows water to exist as a liquid at a temperature lower than 0°C. This is the important part that’s relevant to beer coolers. We’ll come back to this later.
For completeness let’s look at this from the perspective of melting point. When salt is added to ice it lowers the melting point. In other words the ice begins melting at a temperature lower than 0°C. This is why salt is added to ice on the roads in the winter. It causes ice, that would have otherwise remained as a solid in sub-zero temperatures, to turn to water. Note that the temperature of the water has not changed.It’s still at a sub-zero temperature but, as mentioned above, the salt allows it to remain as a liquid at the lower temperature.
The principle of salt lowering the freezing point of water is used frequently to keep roads safe in winter. During snow and ice events, trucks spread a thin layer of salt on roadways. This causes snow and ice to melt on impact rather than freeze and makes the roads wet rather than icy and dangerous. However, there is a limit to how cold water can become before freezing; in extremely frigid temperatures, applying sand to the roads to increase friction is more useful than applying salt. Types of salt other than sodium chloride can be used in colder temperatures. Calcium chloride and magnesium chloride, for example, can melt ice at low temperatures.
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