Vapor Pressure
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MCAT Physical › Vapor Pressure
Which statement is false with regard to vapor pressure?
Diethyl ether has a lower vapor pressure than water due to its greater molecular weight
The boiling point occurs when the vapor pressure of a liquid equals the local atmospheric pressure
Substances with stronger intermolecular forces tend to have lower vapor pressures
Vapor pressure occurs in a state of dynamic equilibrium in which the rate of liquid converting to gas equals the rate of gas converting to liquid
Vapor pressure decreases when a non-volatile solute is added to a pure solvent
Explanation
Vapor pressure represents a state of dynamic equilibrium in which the rate of liquid escaping to gas is equal to the rate of gas condensing to liquid. Liquids with stronger intermolecular forces have a smaller amount of liquid escaping to gas, and thus have a lower vapor pressure.
Diethyl ether does not create hydrogen bonds, whereas water does. Even though diethyl ether has a greater molecular weight, it will have weaker intermolecular forces. The hydrogen bonding interactions in water will cause molecules to "stick," preventing them from converting to the gas phase and lowering the overall vapor pressure of water. Diethyl ether has a higher vapor pressure than water due to the intermolecular forces of the two compounds.
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 vapor pressure above a solvent is 
Cannot be determined from the given information
Explanation
Vapor pressure change is calculated using the following equation.
where 
The vapor pressure of pure solvent is 
This means that the vapor pressure decreased by 
Note that vapor pressure also depends on the amount of ions.
where 
The addition of a nonvolatile solute to a solvent will raise its boiling point. This is because __________.
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 vapor 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
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.
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, 
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.
decrease
increase
either decrease or increase
remain unchanged
be elevated only at low pressures
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.
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?
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
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
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.
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).
Upon addition of salt, a researcher notices that there is an increase in the boiling point of the solution. Which of the following could be the reason for this observation?
The pressure above the solution is lower than the atmospheric pressure
The pressure above the solution is higher than the atmospheric pressure
The amount of particles escaping the solution is higher
The enthalpy of the solution increases
Explanation
The boiling point is defined as the temperature at which the vapor pressure above the solution equals the atmospheric pressure. Recall that vapor pressure is also a colligative property. The vapor pressure decreases as solutes are added. This means that more energy, in the form of heat, is required to increase the amount of molecules escaping the solution and, subsequently, increase the vapor pressure to that of the atmospheric pressure. This increased demand of energy results in an increased boiling point.
A volatile solute with a vapor pressure of 80mmHg is added to a solvent with a vapor pressure of 120mmHg. Consider the resulting solution to be ideal in nature. The solution's vapor pressure is determined to be 93mmHg. What is the percentage of solute molecules found in the solution?
Explanation
A volatile solute will contribute to the vapor pressure found in the container. As a result, we have to use Raoult's law, which takes this solute's vapor pressure into consideration.
Raoult's law is written as 
We must include the vapor pressures of both the solute and the solvent in order to find the percentage composition of the solute in the solution. Since there are only two compounds contributing to vapor pressure in the solution, we can designate the mole fraction of the solute as X and the mole fraction of the solvent as (1-X). Doing this, the equation can be filled in, as below.
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, 
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?
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, 
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?
Vapor pressure may either increase or decrease compared to the pure solvent
Vapor pressure must increase compared to the pure solvent
Vapor pressure must decrease compared to the pure solvent
Vapor pressure will remain unchanged compared to the pure solvent
A volatile solute will not dissolve into solution
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.
When a created solution is either exothermic or endothermic, the vapor pressure in the container will deviate from Raoult's law. As a result, the solution is considered non-ideal.
When a solute is added to a solvent, the vapor pressure of the solution is greater than the vapor pressure of either pure solute or pure solvent. Based on this, which statement is true?
There is a positive enthalpy of solution
The solution is considered ideal
The solution is exothermic
The intermolecular bonds in solution are stronger than the intermolecular bonds between the pure substances
Explanation
Raoult's law can be written as 
When the formation of a solution has a positive enthalpy, it is considered to be endothermic. An endothermic reaction will result in the solution's vapor pressure being higher than predicted by Raoult's law. This is because an endothermic reaction results in weaker intermolecular bonds, which increases the vapor pressure.