Chemistry problems

[Aganack1]

ok so i have these two chemisty problems that I'm stugling with. so if anyone can explain how to get the answer i would be very greatful.

Which one of the followings would have the lowest vapor pressure?

a. 0.1M C12H22O11
b. 0.1M NaCl
c. 0.1M CH3OH
d. 0.1M MgCl2
e. Water

Which one would have the highest boiling point?
a. 0.1M C12H22O11
b. 0.1M NaCl
c. 0.1M CH3OH
d. 0.1M MgCl2
e. Water

I doubt the poster still cares after 10 years.
admin allisolm

 

[Kyteland]

I am not a chemist so don't take my answers as correct.

A) is sucrose (a sugar)
B) is table salt (a salt)
C) methanol (an alcholol)
D) don't know the name (a salt)
E) water (water, duh)

NaCl and MgCl2 are both salts, so I would guess that one of them would have the lowest vapor pressure as well as the highest boiling point. You can probably look these up in your chemistry book. Maybe I'm wrong and the sugar is the answer.

Edit:
http://en.wikipedia.org/wiki/Sucrose
http://en.wikipedia.org/wiki/Sodium_chloride
http://en.wikipedia.org/wiki/Methanol
http://en.wikipedia.org/wiki/Magnesium_chloride
http://en.wikipedia.org/wiki/H2o

NaCl Boiling point 1738 K (1465 °C)
Methanol Boiling point 337.8 K (64.7°C)
MgCl2 Boiling point 1412 °C (1685 K)

 

[Veramocor]

Second question I would say D. The more solids dissolved in water the higher the boiling point rise. Water would hense have the lowest BP since it has nothing dissolved in it. For the other four options you have .1 mole each. But the salts dissasociate to Na+ and CL- and Mg 2+ and 2Cl-. Thus the Magchloride salt would raise the boiling point becuase you have more things individually dissolved in the water.

 

[intogamer]

 

[aplefka]

Haven't had chemistry for a while. Sorry bro.

 

[Kyteland]

Oh, molar solutions.

Nevermind then.

 

[Veramocor]

Also the first question is 'water'. The higher the vapor pressure the higher the boiling point. Adding anything to the water will raise its boiling point and thus its vapor pressure.

So the lowest vapor pressure would water without anything in it. (E)

Edit: Methanol is a tempting answer too. Because it isn't a solid dissolved in water like the other answers. I think methanol boils at a lower pressure than water. So this might be the answer too.

Double Edit: Or maybe I don't know what I'm talking about.

 

[Old_Doc]

Also the first question is 'water'. The higher the vapor pressure the higher the boiling point. Adding anything to the water will raise its boiling point and thus its vapor pressure.

So the lowest vapor pressure would water without anything in it. (E)

Edit: Methanol is a tempting answer too. Because it isn't a solid dissolved in water like the other answers. I think methanol boils at a lower pressure than water. So this might be the answer too.

Double Edit: Or maybe I don't know what I'm talking about.

____________________________________________________________
I thought vapor pressure was inverse relative to boiling point. Decreasing VP means that molecules are being held in the liquid continuum with a greater interactive force than liquids with higher vapor pressures. The higher binding forces would require higher energy to transition molecules into the vapor phase and reach the VP required for boiling, which is equal to the surrounding atmospheric pressure.

As far as your problem goes... Methanol will provide the greater particle to particle interaction because of Hydrogen Bonding and therefore will cause the greater decrease in VP of the options given. Also, for the second question, because methanol provides Hydrogen Bonding and decreases VP more than the other options, solution containing methanol would therefore have the higher boiling points.

As for the choices given, C12H22O11 provides no significant particle - particle interaction and therefore does little to decrease VP and increase BPt.

NaCl => Na(+) + Cl(-) => 0.20M increase in particle numbers and MgCl2 => Mg(+) + 2Cl(-) => 0.30M increase in particle numbers. Both salts will provide some increase in particle - particle interaction, however, the amount of decrease in VP and increase in BPt is miniscule compared to the effect of Hydrogen Bonding.

Water, in itself, is the reference/control liquid whose VP and BPt changes with respect to the amounts and types of solutes added. Since there is no solute added, then water would therefore have the highest VP and lowest BPt of the options given.

In all of this, I would suggest reviewing the effect of Hydrogen Bonding on solution vapor pressure and boiling points. Also, a review of Colligative Properties of solutions would help. Specifically, see the concepts boiling point elevation and freezing point depression as it applies to preparation and characterization of solutions. Ya'll have a great day.

 

[Paperdoc]

I have a sneaking suspicion that the rough "rule of thumb" here is in the NUMBER of dissolved moieties or particles in the solution. Among the five solutions, water (no solute) has no dissolved solute particles, sugar yields one mole of particles (molecules) per mole of solute, methanol will do almost the same, NaCl will produce two ions per molecule, and MgCl2 will produce three ions per molecule. Thus I predict that the MgCl2 0.1M solution will have the largest number of dissolved particles (ions, in this case) and that will produce the largest reduction of vapor pressure of the solution. This also means that the boiling point (that is, the temperature to which the solution must be heated to get it vapor pressure equal to atmospheric pressure) will be highest for that 0.1M MgCl2 solution.

One way to check this is to look for a table of Molal Boiling Point Elevation Constants. It is known that for water-based solutions, almost every solute you can try has a unique constant value for the magnitude of the rise of Boiling Point (above the BP of plain water, 100C) that solute produces, along the lines of:

BP Rise = (Solute concentration in Molality units) x Solute's Molal BP Elevation Constant

Tables of these values are published for many solutes.