Answer to Question #261725 in General Chemistry for leo

Question

An aqueous solution contains 3.00% phenylalanine (C9H11NO2) by mass. Assume the phenylalanine is non-ionic and nonvolatile. What is the freezing point of the solution in degrees Celsius

Answer 1

Anyway, the decrease in the freezing point of a solution is directly proportional to the molality of the solute:

 

ΔT_f = K_fm

 

K_f is 1.86 °C/m for aqueous solution.

 

If the concentration of phenylalanine is in mass percent, then the molality can be found like this:

 

(Note: for some reason, the underlining I did to try to make the molality calculation look more like a real math equation doesn't seem to be showing up. So, there is supposed to be a division line between the 3.00 g of phenylalanine and 97.00 g of water, for example)

 

3.00% phenylalanine = (3 grams of phenylalanine)/(100 grams of solution)

 

Molality = (moles of phenylalanine)/(kilograms water)

      

      = 3.00 g of phenylalanine  *  1mol phenylalanine   *  1000 g water

           97.00 g water       165.0 g phenylalanine   1 kg water

 

      = 0.187 mol phenylalanine/kg water = 0.187 m

 

This means that ΔT_f = (1.86 °C/m)(0.187 m) = 0.349 °C

 

The normal freezing point of water is 0°C, so the new freezing point is -0.349 °C