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Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution.
If a polar molecule were added to the solution, how would the slope of segment AD change? Possible Answers:
It will approach a horizontal asymptote
It will become more negative
It will become less negative
Correct answer: It will become less negative Explanation: This question asks us to consider what would happen to the freezing point (also known as the melting point) of the solution if a polar molecule were added, for exampl adding NaCl to water. More directly, this question is a freezing point elevation/depression problem. After we determine what would happen to the freezing point of the solution, we can predict the change in the slope of the line. The formula for freezing point depression is given below. kf is the cryoscopic constant that says how easily a solution freezes with a given change in temperature, m is the molality of the solution, and i is the Vant Hoff factor. (The Vant Hoff factor is the number of ion particles per individual molecule that has been added; i.e. for NaCl this is 2 because it will separate into exactly two ionic particles when dissociated). Given that kf and b will be constants for a given solution, we know that adding a molecule will decrease the freezing point because i has some positive value. Now that we know the freezing point is depressed (lowered), how will the slope of the line change? At the same pressure, a lower temperature will be required for the solution with the added polar molecule to freeze, thus the slope will be less negative (segment AD will look more flat on the graph). Why does the added molecule have to be polar to have an effect? First, notice that we are told that the phase diagram is of a polar solution. Next, let’s look at the i factor. i only has a value if the added molecules can be dissolved in the original solution. If, for example, we added a nonpolar molecule, it could not be dissolved by the solution and will not contribute to depressing the freezing point. “Like-dissolves-like” also applies here; the freezing point will only change if the solution and molecule added are both polar or both nonpolar.
A 2-liter aqueous solution of glucose has a freezing point of .
Given the solution's freezing point, how many moles of glucose were added to the water? Possible Answers:
Correct answer: Explanation: Since we know that the freezing point of the water has changed by 3.5 degrees, we can solve for how many moles of glucose were added using the freezing point depression equation: Since glucose does not ionize in solution, the vant Hoff factor is equal to 1. Keep in mind that we must use molality for the concentration.
Colligative properties are properties of compounds that are altered by the amount of substance present. There are four main colligative properties: boiling point, freezing point, vapor pressure, and osmotic pressure. The change in each of these properties can be calculated using the amount of molecules/ions present in solution and the concentration or partial pressure of the compound. The boiling point is defined as the temperature at which the vapor pressure equals the atmospheric pressure. The freezing point is the temperature at which a liquid is converted to a solid. Vapor pressure is the pressure produced by the vapor above a solution. Osmotic pressure is the pressure required to prevent flow of water into a solution (across a membrane).
The freezing point of a solution __________ and the boiling point of the solution __________ after the addition of . Possible Answers:
decreases . . . decreases
increases . . . decreases
increases . . . increases
decreases . . . increases
Correct answer: decreases . . . increases Explanation: Freezing point and boiling point are both colligative properties that are altered by the addition of solutes. Addition of solutes decreases freezing point whereas addition of solutes increases boiling point. These phenomena are called freezing point depression and boiling point elevation, respectively. The other two colligative properties are vapor pressure and osmotic pressure. Vapor pressure decreases and osmotic pressure increases after addition of solutes.
Colligative properties are properties of compounds that are altered by the amount of substance present. There are four main colligative properties: boiling point, freezing point, vapor pressure, and osmotic pressure. The change in each of these properties can be calculated using the amount of molecules/ions present in solution and the concentration or partial pressure of the compound. The boiling point is defined as the temperature at which the vapor pressure equals the atmospheric pressure. The freezing point is the temperature at which a liquid is converted to a solid. Vapor pressure is the pressure produced by the vapor above a solution. Osmotic pressure is the pressure required to prevent flow of water into a solution (across a membrane).
Consider the following two solutions: Solution A: Solution B: Which of the following is true regarding these two solutions? Possible Answers:
Solution B has 3 times higher freezing point
Solution B has 2 times higher freezing point
Solution A has 3 times higher freezing point
Solution A has 2 times higher freezing point
Correct answer: Solution A has 3 times higher freezing point Explanation: The freezing point depression can be calculated by the following equation. where is change in boiling point, is the freezing point depression constant, is the number of ions, and is the molality. Solution B will produce three ions whereas solution A will produce two ions in solution; therefore, if the concentration were the same, solution B will have a lower freezing point by a factor of 1.5. The concentration, however, is different. The concentration of solution B is twice as much as solution A; therefore, the freezing point of solution B will be lower by a factor of 3 (1.5 from ions and 2 from concentration); therefore, solution A will have 3 times higher freezing point.
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils. Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here. In this law, is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.
If a non-volatile solute is added to a solvent, the freezing point of the solution tends to __________ relative to the pure solvent's freezing point. Possible Answers:
be elevated only at low pressures
either decrease or increase
Explanation: Freezing points are decreased, or depressed, with the addition of non-volatile solutes in a similar manner to boiling point elevation. The addition of a solute makes phase changes more difficult, and thus solutions with non-volatile solutes require more heat to boil, or a colder environment to freeze. The solute ions and particles in solution disrupt the forces between solvent molecules, preventing the formation of a solid frozen lattice.
The addition of a nonvolatile solute to a solvent will raise its boiling point. This is because __________. Possible Answers:
the solute increases the vapor pressure
the solute causes more heat to be needed in order for the solution to have the same vapor pressure as the atmospheric pressure
the solute increases the heat capacity of the solvent
the solute reacts with the solvent and creates a product more resistant to temperature change
Correct answer: the solute causes more heat to be needed in order for the solution to have the same vapor pressure as the atmospheric pressure Explanation: Remember that a nonvolatile solute will lower the vapor pressure of a solvent in proportion to the mole fraction of the solvent (Raoult's Law). Since vapor pressure must equal the atmospheric pressure in order to boil, a greater amount of heat is required to increase the lowered vapor pressure of the solution.
A nonvolatile solute is added to a solution so that it makes up 5% of the molecules in the solution. Which of the following is true? Possible Answers:
The vapor pressure is unaffected by the addition of the solute
The vapor pressure of the solution is greater than the pure solvent's vapor pressure
The solution's vapor pressure is less than 95% of the pure solvent's vapor pressure
The solution has 95% of the vapor pressure of the pure solvent
The solution's vapor pressure is somewhere between 95% and 100% of the pure solvent's vapor pressure
Correct answer: The solution has 95% of the vapor pressure of the pure solvent Explanation: When a nonvolatile solute is added to a solvent, it will not contribute to the molecules which exert pressure on the container. It will, however, take up some of the surface area interacting with the air in the container. This reduces the number of solvent molecules that are able to break from their bonds and become gas molecules in the container. The reduction of vapor pressure is dependent on the percentage of solute molecules in the solution. Since 5% of the molecules in this solution come from the solute, the vapor pressure will be 95% of the pure solvent's vapor pressure.
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils. Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here. In this law, is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.
A scientist is studying an unknown solution with a non-volatile solute, and determines that the solute has a mole fraction of 0.36. The original solvent has a vapor pressure of 2atm. What is the vapor pressure of the solution being studied? Possible Answers:
Correct answer: Explanation: The question gives us the mole fraction of solute, not solvent, thus, we need to realize that the mole fraction of solvent is the difference between one and the given value. Now we can use Raoult's Law to find the solution vapor pressure.
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils. Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here. In this law, is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.
A scientist is testing Raoult's Law, but accidently adds a volatile solute instead of a non-volatile solute. Which of the following is true? Possible Answers:
Vapor pressure must decrease compared to the pure solvent
Vapor pressure must increase compared to the pure solvent
Vapor pressure may either increase or decrease compared to the pure solvent
Vapor pressure will remain unchanged compared to the pure solvent
A volatile solute will not dissolve into solution
Correct answer: Vapor pressure may either increase or decrease compared to the pure solvent Explanation: A volatile solute has its own vapor pressure. As a result, it may lead to a higher total vapor pressure than the solvent in isolation. Since we are not told the vapor pressure of the volatile solute, it can either raise or lower the total vapor pressure. This will depend on the comparison of the vapor pressure of the pure solute and that of the pure solvent. If the solute has higher vapor pressure, then adding it to the solvent will raise the vapor pressure. If the solvent has higher vapor pressure, then adding solute will lower the vapor pressure.
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils. Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here. In this law, is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.
How would you best modify Raoult's Law to find the total vapor pressure of a solution with a volatile solute? Possible Answers:
Correct answer: Explanation: You would add the contribution of the solute to the total vapor pressure of the solvent. Since the solute is contributing to the total vapor pressure of the solution, it must simply be added to the solvent vapor pressure.
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