Problem: A 50.0-mL solution is initially 1.54% MgCl2 by mass and has a density of 1.05 g/mL.What is the freezing point of the solution after you add an additional 1.34 g MgCl2? (Use i = 2.5 for MgCl2.)

FREE Expert Solution

We’re being asked to find the freezing point of a 50.0- mL 1.54 % MgCl2 by mass solution after adding an additional 1.34 gMgCl2 ( i = 2.5 for MgCl2.)


Recall that the freezing point of a solution is lower than the freezing point of a pure solvent.


When calculating the freezing point of a solution, we’re going to use the equation for Freezing Point Depression:


Tf=i×Kf×m


∆Tf = change in freezing point = Tf pure solvent –Tf solution
Kf = freezing point depression constant
i = van' t Hoff factor of the solute = no. of ions
m = molality


We need to find the molality of the solution after adding additional MgCl2 to solve for its freezing point by doing these steps: 

      

Step 1. Determine the composition of the solution after adding MgCl2.


Since we're given the volume and density of the solution, we can determine its mass using the equation:


density= massvolume


Also, recall that Mass or weight percent (% by mass) is the percentage of a given element or compound within a solution. 


The equation used to calculate for mass percent is shown below:


Mass component mass of solute
• Total mass  mass of solution


mass percent = mass componenttotal mass×100


Step 2. Calculate the moles of the solute


Step 3. Calculate the molality of the solution.


molality = mol solutekg solvent

Solute = MgCl­2 
Solvent = Water (H2O)


Step 4. Calculate the freezing point of the solution.


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Problem Details

A 50.0-mL solution is initially 1.54% MgCl2 by mass and has a density of 1.05 g/mL.

What is the freezing point of the solution after you add an additional 1.34 g MgCl2? (Use i = 2.5 for MgCl2.)

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Freezing Point Depression concept. If you need more Freezing Point Depression practice, you can also practice Freezing Point Depression practice problems.

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