Molar Mass From Freezing Point Depression

In summary, the nonpolar molecular compound will freeze at -1.87 C when dissolved in benzene if 5.52 grams is dissolved. The freezing point depression constant is -5.12 and the molar mass is 106.48 grams per mole.
  • #1
TrueStar
95
0

Homework Statement



Find the molar mass of a nonpolar molecular compound if 5.52 grams dissolved in 36.0 grams of benzene begins to freeze at -1.87 C? The freezing point of pure benzene is 5.50 C. The freezing point depression constant is -5.12.

Homework Equations



(Change of temperature)=(FP constant)(molality)
MM=moles/gram
m=moles of solute/kg of solvent


The Attempt at a Solution



The change in temperature is 5.50-(-1.87) which is 7.37.
7.37=5.12x --- x will equal .07. This equates to .07 moles of solute per solvent.

I'm not sure what to do next to get the molar mass of the compound. Would I divide 5.52 by .07 moles?

Thank you!
 
Physics news on Phys.org
  • #2
I haven't checked your numbers, but

Molar mass = grams/moles. Divide grams by moles.
 
  • #3
n=m/M then n/1=m/M then m=n x m
 
  • #4
I know molar mass is grams/mole. I just am not sure if it's OK to divide the 5.16 grams by the .07 moles because it's actually .07 moles per Kg. (molality).
 
  • #5
You doubts are not unfounded. You don't have 0.07 moles of substance - that you would have in 1000 g of bezene. But there is only 36 g of benzene, so obviously number of moles is much smaller.

--
methods
 
  • #6
OK, here's what I've done on the second try.

The problem setup with the freezing point depression equation is 7.37=5.12x. x will equal to 1.44 which means 1.44 moles of solute per kg of solvent. I don't have that much solvent, just 36g. After changing kg to grams and then multiplying that number of moles by 36, I have .05184 moles.

I know there is 5.52 g of solvent in the solution. That's 5.52 g per .05184 moles. That translations to 106.48 g per mole...which is the molar mass and the answer.

Am I doing this correctly?
 
  • #7
Looks OK to me.

Beware - you have solved the same equation twice (7.37=5.12x) getting two different results - 0.07 and 1.44. that's not good.

--
methods
 
  • #8
Yes I noticed that in my first post. 1.44 should be the right answer. I divided the wrong way last night. I was tired.
 

FAQ: Molar Mass From Freezing Point Depression

What is molar mass from freezing point depression?

Molar mass from freezing point depression is a scientific concept that describes the relationship between the freezing point of a solution and the concentration and molecular weight of the solute in the solution. It is commonly used in chemistry to determine the molar mass of an unknown solute.

How is molar mass from freezing point depression calculated?

Molar mass from freezing point depression is calculated using the equation: ΔT = Kf * m, where ΔT is the change in freezing point, Kf is the molal freezing point depression constant, and m is the molality of the solution. The molar mass of the solute can then be determined by rearranging the equation to solve for the molar mass (M = Kf * m / ΔT).

What is the molal freezing point depression constant?

The molal freezing point depression constant (Kf) is a physical constant that is specific to a particular solvent and is used to calculate the change in freezing point of a solution. It is determined experimentally and can vary depending on the solvent being used.

How does molar mass from freezing point depression differ from other methods of determining molar mass?

Molar mass from freezing point depression is a colligative property, meaning it is dependent on the number of particles in a solution and not the chemical identity of the solute. This is different from other methods, such as vapor pressure or boiling point elevation, which are also colligative properties but are affected by the chemical identity of the solute.

What are some real-world applications of molar mass from freezing point depression?

Molar mass from freezing point depression is commonly used in industries such as pharmaceuticals, food and beverage, and environmental science. It can be used to determine the purity of a substance, identify unknown substances, and monitor the concentration of solutes in solutions. It is also used in the production of antifreeze and de-icing solutions for vehicles and roads.

Similar threads

Measuring Molecular Mass with Freezing Point Depression
Replies
6
Views
2K
Is freezing point always depressed by a solute?
Replies
3
Views
1K
Composition of Freezing Point Crystals in a Glacial Acetic Acid Solution
Replies
6
Views
2K
Chemistry How to get molality without using a molecular formula?
Replies
1
Views
1K
Colligative Properties, Freezing Point Depression - Chemistry Lab
Replies
1
Views
4K
Delta T is making my solution come out negative
Replies
1
Views
4K
Freezing point depression calculations
Replies
4
Views
5K
Calculate Mass of Non-Volatile Solute in Octane to Reduce V.P. 80%
Replies
1
Views
2K
Calculate freezing point depression and boiling point elevation
Replies
2
Views
16K
Boiling and Freezing Points of Solutions
Replies
2
Views
4K

Hot Threads

Recent Insights

Back
Top