MCAT Physical : Boiling Point
Study concepts, example questions & explanations for MCAT Physical
All MCAT Physical Resources
Example Questions
Example Question #6 : Physical Chemistry
Two moles of sodium chloride (NaCl) are added to 1kg of a mystery solvent. The addition of the NaCl caused an increase of 6K to the solvent's boiling point.
Based on this information, what is the boiling constant for the solvent?
In order to solve this problem, we can use the boiling point elevation equation: 
We know the temperature change, we can compute molality from the given information, and we know the van't Hoff factor (expected to be 2 in this scenario due to NaCl becoming 2 ions in solution). We can calculate the boiling point constant for the solvent.
Example Question #41 : Solution Chemistry
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).
Which of the following is true regarding the boiling point of a sodium chloride solution and calcium chloride solution?
The boiling point of sodium chloride solution will be higher
The boiling point of both solutions will be equal
The boiling point of calcium chloride solution will be higher
The relative boiling points cannot be determined because the concentration is not given
The relative boiling points cannot be determined because the concentration is not given
Recall that the boiling point of a solution depends on the number of ions in the solution and the concentration of the solution. Increase in both of these factors will elevate the boiling point to higher temperatures. The equation describing this is given below.
where 
Example Question #11 : Colligative Properties
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).
How does the boiling point of a solution change after adding 
Increases by
Increases by
Decreases by
Decreases by
Increases by
Recall that adding solutes to a solution increases the boiling point. This phenomenon is called the boiling point elevation. Knowing this information, we can eliminate two choices immediately. The equation to calculate the boiling point elevation is as follows.
where 
Molarity =
Molality =
We need to find the mass of solvent in kilograms. The molarity is 
mass of total solution =
We need the mass of the solvent only. To find this, we need to first calculate the mass of solute. The MW of potassium bromide is 
This means that the mass of solvent is equal to:
mass of solvent = total mass of solution - mass of solute = 
Molality of solution is
Therefore, boiling point increases by 
Certified Tutor
Certified Tutor
All MCAT Physical Resources
