40.0 g of magnesium chloride (MgCl2) in 250.0 g of water at 22 °C.Calculate a) the vapor pressure of this solution, b) the freezing point, c) the boiling point.
I know it is something like this, but I continue to get the incorrect answer.
A) 40.og Mgcl2*(1mol mgcl2/ 95.2 mgcl2)*(3 mol mgcl2/ 1mol MgCl2)= 1.261
250.o H20*(1mol h20/ 18.02h20)= 13.87
(13.87)/(13.87+1.261)=.9166--> .9166(23.80)= 21.81. That's not the correct answer, though.
C) I get 100.8 for Boiling point
B) and negative one for freezing.
Can someone please tell me how to do this correctly?
freezing point.
mols MgCl2 = grams/molar mass = 40.095.2 = about 0.42 (that's estimated--you check ALL these numbers).
m = mols/kg solvet = 0.420/0.250 = about 1.68m
delta T = i*Kf*m
i for MgCl2 = 3
dT = 3*1.86*1.68 = about 9.4
freezing point = 0-9.4 = -9.4.
boiling point:
dT = i*Kb*m
i is same
Kb = 0.51
m is same. Solve for dT and add to 100. I think your error here is you didn't use the i factor of 3.
For the vapor pressure part of the problem it looks ok to me EXCEPT I think you used vapor pressure H2O at 25 C and not 22C. According to my sources, v.p. H2O @ 22C = 19.8 mm Hg.
Thank You Dr.Bob. I have one more question, how exactly do you is "i" 3? is it a given?
Yes and no.
i is the van't Hoff factor and it is the number dissolved particles.
So for NaCl you substitute i = 2.
For MgCl2 it is 3.
For MgSO4 it is 2
For SnCl4 it is 5 etc.
Technically, this number we substitute is a limiting number; i.e., it won't be greater than 3,2,5,etc but it may be less. We usually ignore all of those things that make it less and use the limiting number. When you get into activities you will see how that affects it.
I see it mainly deals with Ionic compounds correct? MgCL2 is two because when it is separated it makes 3 moles of MgCl2 for every one mole of MgCl2?
Yes, except for the typo you made. MgCl2 is THREE because when it is separated it makes 3 mols .....
Covalent compounds have i = 1.
To calculate the properties of a solution, you need to consider the colligative properties, which depend on the number of particles in the solution rather than their identity. The colligative properties being asked here are the vapor pressure lowering, freezing point depression, and boiling point elevation.
a) Vapor Pressure:
The vapor pressure of a solution is given by Raoult's law, which states that the vapor pressure of a component in a solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution.
First, calculate the moles of MgCl2 and water present:
- Moles of MgCl2 = mass of MgCl2 / molar mass of MgCl2
- Moles of H2O = mass of H2O / molar mass of H2O
Then calculate the mole fraction of MgCl2:
- Mole fraction of MgCl2 = moles of MgCl2 / (moles of MgCl2 + moles of H2O)
Finally, calculate the vapor pressure of the solution:
- Vapor pressure = mole fraction of MgCl2 * vapor pressure of pure water
b) Freezing Point:
The freezing point depression is given by the equation ΔTf = Kf * molality, where ΔTf is the change in freezing point, Kf is the cryoscopic constant, and molality is the number of moles of solute per kilogram of solvent.
Calculate the molality of the solution by dividing the moles of MgCl2 by the mass of water in kilograms.
Then calculate the freezing point depression by multiplying the molality by the cryoscopic constant.
The freezing point of the solution is the freezing point of pure water minus the freezing point depression.
c) Boiling Point:
The boiling point elevation is given by the equation ΔTb = Kb * molality, where ΔTb is the change in boiling point, Kb is the ebullioscopic constant, and molality is the number of moles of solute per kilogram of solvent.
Calculate the molality of the solution as done before.
Then calculate the boiling point elevation by multiplying the molality by the ebullioscopic constant.
The boiling point of the solution is the boiling point of pure water plus the boiling point elevation.
Make sure to use the correct constants for water, which are: Kf = 1.86°C/m and Kb = 0.512°C/m.
By following these steps, you should be able to obtain the correct answers for the vapor pressure, freezing point, and boiling point of the given solution.