Colligative Properties

[bamboo44]

Can someone please clarify boiling point elevation and freezing point depression?

Also, here's a question:

Q. The freezing point temperature of the mixed solution in Trial 2 is expected to be:

c) higher than the freezing point temperatures of the initial acid and base solutions.


Info from passage:

Trial 2- At 25 degrees Celsius, 50 ml of 2.0 M NaOH was combined with 50.0 ml of 2.0 M HCl. The mixture temperature was recorded as 71 degrees celsius.


I thought the freezing point of the mixture would be lower than the acid and base solutions....not higher!! Someone please explain..

I feel like it might have to do with the vant Hoff factor....but I'm not sure..

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[Isoprop]

you're pretty much making a more dilute solution when you mix them both.

Step 1. calculate the number of moles of HCl and NaOH you have
2 M*50 mL = 100 mmol
So you have 100 mmol NaOH and HCl.

Step 2. write out the reaction.
100 mmol of NaOH and HCl will combine to form 100 mmol of NaCl and 100 mmol of water

Step 3. write out the new concentration of the mixture
molality = 100 mmol/(50g + 50g) = 1 molals

Step 4. compare the freezing point depressions.
T = -ikm
Since i and k are the same of the original solns and the final solns, we just need to compare m. We started with approximately 2m solns and ended with a 1m soln. So freezing point won't be depressed as much with the final soln.

Note-I neglected to add the 100 mmol of water which is about 2 g of water to the final soln. It makes a negligible difference anyway AND it would only dilute the final solution even more.

 

[fizzgig]

this may be basic but i can't remember seeing it in EK... do OH and H not count as solute? So 1mol NaOH added to water means you added 1mol solute (the Na ions), and OH is not included? Just checking...

 

[Isoprop]

No they do count. So 1 M of NaOH has 2 moles of ions in them.

It's just that when NaOH and HCl combine, the H+ and the OH- will form water, and water isn't a solute.

 

[bamboo44]

Thank you Isoprop.

I have another question....

A scientist is investigating the melting behavior of acetamide. In a beaker, he brings the water to a a slow boil (100 degrees celsius) and a test tube filled with 10g of acetamide crystals were added. At about 80 degrees celsius, the acetamide started melting. So the melting point for acetamide is 80 degrees.

Q. Which of the following would most likely have occurred if the water had only reached 90 degrees celsius before the acetamide would have been placed in?

a) More water would have been used to melt the acetamide
b) Less water would have been used to melt the acetamide
c) The acetamide wouldn't have melted
d) The acetamide would have taken longer to completely melt

The answer is "d" and I'm not sure why. I mean it makes sense logically speaking..but not according to the rules of colligative properties. I actually picked "b" because I thought less water meant more concentrated..so the acetamide would have melted at a lower temperature..which is good because the water is now at a lower temperature.......

Actually.....I just thought of something.....is it because the acetamide is in a TEST TUBE ...so it there is no SOLUTION to begin with.....the test tube and the beaker of water are totally different and do not associate at all...so concentration is of no relevance?? I think I just answered my own question..but someone please confirm!! Thankssssss!!

 

[fizzgig]

bamboo yes i think you are correct.

isoprop thanks for the answer... i think i get what tripped me up (for now anyway )

 

[Moosecakes]

Thank you Isoprop.

I have another question....

A scientist is investigating the melting behavior of acetamide. In a beaker, he brings the water to a a slow boil (100 degrees celsius) and a test tube filled with 10g of acetamide crystals were added. At about 80 degrees celsius, the acetamide started melting. So the melting point for acetamide is 80 degrees.

Q. Which of the following would most likely have occurred if the water had only reached 90 degrees celsius before the acetamide would have been placed in?

a) More water would have been used to melt the acetamide
b) Less water would have been used to melt the acetamide
c) The acetamide wouldn't have melted
d) The acetamide would have taken longer to completely melt

The answer is "d" and I'm not sure why. I mean it makes sense logically speaking..but not according to the rules of colligative properties. I actually picked "b" because I thought less water meant more concentrated..so the acetamide would have melted at a lower temperature..which is good because the water is now at a lower temperature.......

Actually.....I just thought of something.....is it because the acetamide is in a TEST TUBE ...so it there is no SOLUTION to begin with.....the test tube and the beaker of water are totally different and do not associate at all...so concentration is of no relevance?? I think I just answered my own question..but someone please confirm!! Thankssssss!!

Acetamide CRYSTALS. They are adding the water into a beaker, not into the test tube containing the crystals. You can add all the water you want, but if it is at 90* it is going to take just as long, if not longer really (since the freshly added water now needs to reach equilibrium with the already 90* water) to melt the crystals in the test tube. D is the only logical answer there, hope i helped a bit!!