# Chemistry

posted by .

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

Can someone please tell me how to do this correctly?

• Chemistry -

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.

• Chemistry -

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.

• Chemistry -

Thank You Dr.Bob. I have one more question, how exactly do you is "i" 3? is it a given?

• Chemistry -

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.

• Chemistry -

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?

• Chemistry -

Yes, except for the typo you made. MgCl2 is THREE because when it is separated it makes 3 mols .....
Covalent compounds have i = 1.

## Similar Questions

1. ### Chemistry

How does dissolving a solute affect the boiling point or freezing point of the solution?
2. ### Chemistry

A solution is prepared by placing 72.8 g of calcium chloride in 120 g of water at 22C. The vapor pressure of water at 22C is 2.6 kPa. a)What is the boiling point of the solution?
3. ### Chemistry II

A solution is prepared by placing 72.8 g of magnesium chloride in 120 g of water at 22oC. a)What is the boiling point of the solution?
4. ### Chemistry

Calculate the boiling points of freezing points of the following solutions in water. a) boiling point of a 2.25m solution of propylene glycol (a non-electrolyte) b)freezing point of a 1.0m solution of potassium iodide c)freezing point …
5. ### Chemistry

the following questions are about colligative properties: a)How much would the freezing point change in a 2.5 solution of phenol in benzene?

1) In comparing the phase diagram for water with that of a solution of sugar in water, all of the following decrease except a) the vapor pressure b) the boiling point c) the freezing point d) the triple point I chose the boiling point …
7. ### Chemistry

A solution contains 15 g non-volatile non-electrolyte solute C6H14O6 dissolved in 500 g of water at 40 c. Calculate (A)Vapor pressure of the solution. (B)Boiling point of the solution. (C)Freezing point of the solution.
8. ### Chemistry

A solution of magnesium chloride, MgCl2, is prepared by dissolving 19.0g of MgCl2 in 250 mL OF water. a.) Calculate the weight percent of magnesium chloride in solution. b.)The denstiy of the above solution is 1.05 g/mL. Calculate …
9. ### Chemistry

1) What is the vapor pressure at 25°C of a solution of 1.20g ofnephtalene C10H8 in 25.6g of benzone C6H6?
10. ### CHEMISTRY

1. What is the vapor pressure at 25°C of a solution of 1.20g of nephtalene C10H8 in 25.6g of benzone C6H6?

More Similar Questions