Chemistry, colligative properties-freezing point

In summary, the freezing point of the solution can be determined by using the molal freezing point-depression constant of water and the Van't Hoff Factor. The Van't Hoff Factor can be calculated by finding the moles of particles in solution and the moles of solute dissolved. In this case, the solution contains 1.29 mol of K2CrO4 and has a molality of 1.29 mol/kg. The change in freezing point is found to be -2.40 degrees Celsius, which results in a freezing point of -7.18 degrees Celsius for the new solution. It is important to note that the Van't Hoff Factor for ionic compounds is typically 1 since they usually dissociate completely.
  • #1
Flip
8
0
Homework Statement

A solution is made by dissolving 250.0 g of solid potassium chromate in 1.00 kg of water. What will be the freezing point of the new solution?

molal freezing point-depression constant of water = 1.86 degrees Celsius/molality
molality = mol/kg

Relevant equations

[Delta]T(freezing point)=(Van't Hoff Factor)(molal concentration of solute particles)(molal freezing point-depression constant)

Van't Hoff Factor = (moles of particles in solution/moles of solute dissolved)The attempt at a solution

(250 g K2CrO4)(1 mol K2CrO4/194.188 K2CrO4) = 1.29 mol of K2CrO4
(1.29 mol K2CrO4/1.00 kg H2O) = 1.29 mol/kg
Change in freezing point = (1.29 molality)(1.86 degrees Celsius/molality)
Change in freezing point = 0 degrees celsius - 2.40 degrees celsius
Change in freezing point = -2.40 degrees celsiusComments

Answer is -7.18 degrees celsius
Since ionic compounds rarely dissociate completely the Van't Hoff Factor has to be used. Except I don't know how to use it...
 
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  • #2
Flip said:
Since ionic compounds rarely dissociate completely

This is false statement. Simple ionic salts almost always dissociate 100%.

the Van't Hoff Factor has to be used. Except I don't know how to use it...

What is van't Hoff factor for this salt (assuming 100% dissociation)?
 
  • #3
The Van't Hoff Factor isn't given, which is why I thought that I had to find it myself. But if it does dissociate completely then doesn't that mean that the Van't Hoff Factor would just be 1?
 
  • #4
No. Write equation of dissociation reaction and use the definition you have already posted.
 

What is colligative properties-freezing point?

Colligative properties are physical properties of a solution that depend on the concentration of particles, rather than the specific type of particle. Freezing point is one such colligative property, which is the temperature at which a liquid will turn into a solid.

What factors affect the freezing point of a solution?

The freezing point of a solution is affected by the concentration of particles in the solution. The more particles present, the lower the freezing point will be. Additionally, the type of particles in the solution can also affect the freezing point.

How is the freezing point of a solution measured in a laboratory?

The freezing point of a solution can be measured using a device called a cryoscope. This device uses the principle of freezing point depression, where a solution with a lower freezing point will cause the temperature to drop more when it freezes, allowing for an accurate measurement.

What is the purpose of studying colligative properties-freezing point?

Studying colligative properties, including freezing point, can give scientists insight into the behavior of solutions. It can also be used to determine the concentration of particles in a solution, which is important in various industries such as food science and pharmaceuticals.

How can colligative properties-freezing point be applied in real life?

Colligative properties, including freezing point, have many practical applications in everyday life. For example, adding salt to icy roads lowers the freezing point of water, preventing the roads from freezing. In food preservation, adding sugar to jams and jellies lowers the freezing point, making them less likely to spoil. In addition, the study of colligative properties can help in developing more effective antifreeze solutions for vehicles.

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