Question

solution is made by dissolving 25.5 grams of glucose (C6H12O6) in 398 grams of water. What is the freezing-point depression of the solvent if the freezing point constant is -1.86 °C/m? Show all of the work needed to solve this problem.

Answer 1

So for this one we have a couple extra steps so first we have to find the number of moles of glucose to do this:
n=m/MM
moles= mass / molar mass
So mass is the 25.5g and the molar mass of glucose is 180.16
So 25.5/180.16=n
n=?

Answer 2

n=0.142

Answer 3

Great! So next step we need to find moles per kg of water but first we have to convert 398g into kg so you divide by 1000 as that is the conversion

Answer 4

0.398

Answer 5

Yes so now we can figure out the moles per kg
m=0.142/0.398
m=?

Answer 6

0.357

Answer 7

Yes 0.357moles/kg, so the next step is the van't Hoff factor which again is represented by i, it is a dissociation factor, glucose doesn't dissociate in water so i=1 for this example

Answer 8

Second last step now is to find our change in temperature:
T=iKfm
i is 1, Kf is given so -1.86 and m is the 0.357 we found previously

Answer 9

T=(1)(-1.86)(0.357) = -0.664

Answer 10

Yes so that is the change in temperature when the glucose is mixed in so to find the freezing point we add this to the freezing point of water
So 0+(-0.664)= -0.664 celcius is the freezing point of this reaction

Answer 11

why 0?

Answer 12

Because 0 is the freezing point of water

Answer 13

When we did the boiling point we used 100 as that is the boiling point of water

Answer 14

ok so the answer is -0.664?

Answer 15

Yes it would be

Answer 16

thx

Answer 17

No problem! Happy to help!!