MCAT Physical : Vapor Pressure
Study concepts, example questions & explanations for MCAT Physical
All MCAT Physical Resources
Example Questions
Example Question #1 : 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, 
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
remain unchanged
either decrease or increase
be elevated only at low pressures
increase
decrease
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.
Example Question #21 : Colligative Properties
The addition of a nonvolatile solute to a solvent will raise its boiling point. This is because __________.
the solute increases the heat capacity of the solvent
the solute reacts with the solvent and creates a product more resistant to temperature change
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 causes more heat to be needed in order for the solution to have the same vapor pressure as the atmospheric pressure
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.
Example Question #1 : Vapor Pressure
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'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 vapor pressure of the solution is greater than the pure solvent's vapor pressure
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 solution has 95% of the vapor pressure of the pure solvent
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.
Example Question #61 : Solution Chemistry
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?
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.
Example Question #3 : 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 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
Vapor pressure must increase compared to the pure solvent
Vapor pressure may either increase or decrease compared to the pure solvent
Vapor pressure may either increase or decrease compared to the pure solvent
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.
Example Question #4 : 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, 
How would you best modify Raoult's Law to find the total vapor pressure of a solution with a volatile solute?
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.
Example Question #1 : Vapor Pressure
Which statement is false with regard to vapor pressure?
The boiling point occurs when the vapor pressure of a liquid equals the local atmospheric pressure
Vapor pressure decreases when a non-volatile solute is added to a pure solvent
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
Diethyl ether has a lower vapor pressure than water due to its greater molecular weight
Substances with stronger intermolecular forces tend to have lower vapor pressures
Diethyl ether has a lower vapor pressure than water due to its greater molecular weight
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.
Example Question #62 : Solution Chemistry
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?
The solution is considered ideal
The solution is exothermic
There is a positive enthalpy of solution
The intermolecular bonds in solution are stronger than the intermolecular bonds between the pure substances
There is a positive enthalpy of solution
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.
Example Question #1 : Vapor Pressure
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?
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.
Example Question #31 : Colligative Properties
At room temperature, toluene has a vapor pressure of 300mmHg and ethanol has a vapor pressure of 45mmHg.
Let us assume that when equimolar amounts of these liquids are mixed together, heat is released. What can be said about the vapor pressure of the resulting solution?
The vapor pressure will be 172.5 mmHg because there are equal amounts of both liquids
The vapor pressure is less than 172.5 because the reaction is exothermic
The vapor pressure will be 300 mmHg because toluene has the greater vapor pressure, which cancels out the vapor pressure of ethanol
The vapor pressure will be 345 mmHg because the vapor pressure values are additive
The vapor pressure is less than 172.5 because the reaction is exothermic
Since both liquids are volatile, they will both contribute to the solution's vapor pressure. Their impact will be based on their individual vapor pressures, as well as the molar fraction for which each liquid is responsible. This gives us Raoult's law:
Each 
This would result in a solution vapor pressure of 172.5 mmHg if conditions were ideal. However, since the reaction is exothermic, stronger bonds are formed in the solution, and the vapor pressure will be lower than expected.
Certified Tutor
Certified Tutor
All MCAT Physical Resources
