Gen Chem Question

[Phenotype]

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Which of the following has the lowest Boiling point?

a) Diamond
b) KNO3
c) C3H8
d) C3H5(OH)3
e) Cobalt

Ans : C

Another one,
Which solution has the lowest freezing point?

a) 0.2M KCl
b) 1.2M C6H12O6
c) 1.4M Na2SO4
d) 2.4M C2H5OH
e) 1.8M NaNO3

Ans: C


How come the first question says it's C3H8 b/c there is only dipole interaction whereas the second question says it's Na2SO4 due to colligative properties?
I got the first question right, but then I thought the second question would be "B" since it has the weakest interaction.

Vapor Pressure, Boiling Point, Osmotic Pressure, Freezing point are examples of colligative properties given; does this mean solubility is not a colligative property?

Question got long, so in bottom line, What is colligative property and why does it only applies to certain conditions?

Could someone explain this plz? Thanks @_@


Thanks!

 

[chromosome21]

Which of the following has the lowest Boiling point?

a) Diamond
b) KNO3
c) C3H8
d) C3H5(OH)3
e) Cobalt

Ans : C
Diamond is made up of "covalent-covalent" bond.. and it'd take so much energy to break such a bond.

KNO3 is made up of K+ and NO3-.. it means it's an Ionic bond.. very strong and it'd take so much energy to break such a bond

C3H5(OH)3.. the OH will make H-bonds.. they are actually inter-molecular attraction forces. Need high energy to break such an interaction

Cobalt is a metal.. and it's made up of metal-metal bonds.. which are stronger than covalent intermolecular forces

The answer, choice C, C3H8 only has Dispersion force, and therefore, it'd have lowest boiling point, and highest vapor pressure.

Another one,
Which solution has the lowest freezing point?

a) 0.2M KCl
b) 1.2M C6H12O6
c) 1.4M Na2SO4
d) 2.4M C2H5OH
e) 1.8M NaNO3

Ans: C
Colligative properties depend on the "# of solutes" and NOT the "type" of solutes. Are you sure the choices are in "M" and not "m".. M is a symbol of Molarity, and m is a symbol for molality.. you should know the differences b/w the two. for colligative properties we use "m" not "M". I'll consider "m" not "M" to make it easier for you.

Freezing point depression change in temp = - k * i * m
negative sign is b/c of the "depression"
assuming that the solvent is the same for all the solutes listed here.
and "m" is given.. so we should simply multiply to easily compare all the choices

a. KCl is a very soluble solute, i = 2, m = 0.2 .. i*m = 0.4
b. c6H12O6 is a sugar and will not dissociate completely.. therefore, i = 1; m = 1.2 .. so, i*m = 1.2
note: you should know what is soluble and what is not soluble.. so I will skip one step of telling you if the solute is soluble or not
c. i = 3 (2Na+, SO4 2-); m = 1.4... i*m = 4.2
d. i = 1, m = 2.4.. i*m = 2.4
e. i = 2, m = 1.8 .. i*m = 3.6

out of all the choices, C has the highest # of solutes, therefore, it'll lower the freezing point the most!


How come the first question says it's C3H8 b/c there is only dipole interaction whereas the second question says it's Na2SO4 due to colligative properties?
I got the first question right, but then I thought the second question would be "B" since it has the weakest interaction.

Vapor Pressure, Boiling Point, Osmotic Pressure, Freezing point are examples of colligative properties given; does this mean solubility is not a colligative property?
nope! solubility is under "equilibrium" chapter. Make sure you are clear on all this. They are important concepts. We're responsible for only those four coligative properties you've mentioned. KNOW them, including calculation.

Question got long, so in bottom line, What is colligative property and why does it only applies to certain conditions?
read #2 explanation.

Could someone explain this plz? Thanks @_@


Thanks!

.

 

[Phenotype]

Then, does it mean the answer to the first question would change if they were all 1.0M solutions? For example,

Which of the following has the lowest Boiling point?

a) 1.0M Diamond
b) 1.0M KNO3
c) 1.0M C3H8
d) 1.0M C3H5(OH)3
e) 1.0M Cobalt

What would the answer be?

 

[sbdento]

Then, does it mean the answer to the first question would change if they were all 1.0M solutions? For example,

Which of the following has the lowest Boiling point?

a) 1.0M Diamond
b) 1.0M KNO3
c) 1.0M C3H8
d) 1.0M C3H5(OH)3
e) 1.0M Cobalt

What would the answer be?

Adding ions will elevate the boiling point and depress the melting point due to the ions getting inbetween the water molecules and not allowing them to do what they want. The ions will "stick" to the water and keep it from evaporating and will "stick" to the water and keep it from freezing.

B is the only one that will dissolve in water, so it is the only one that will have an elevated bp. The other solutions will not be affected by the added compounds, so all of the above except B would be the answer.

Maybe, my non-scientific explanation will help you 🙂

 

[Phenotype]

Which of the following has the lowest Boiling point?

a) 1.0M Diamond
b) 1.0M KNO3
c) 1.0M C3H8
d) 1.0M C3H5(OH)3
e) 1.0M Cobalt

Very helpful explanation! I think I understand how only the ion forming compounds will lower the freezing point and elevate the boiling point. But which of the above would have the LOWEST boiling point?

 

[chromosome21]

what source/material are you using for g.chem? You might want to review these concepts from your main source. It seems like you are not understanding the difference b/w bp vs. bp elevation, fp vs. fp depression.. collegative properties include "bp elevation, fp depression, vp depression, osmotic pressure"... It'd take me a while to write down all the definitions and explain to you in detail, may be some other sdners would do a better job here, but if you tell me what book you are using for g.chem, I could able to guide you better.

-----------
Which of the following has the lowest Boiling point?

a) 1.0M Diamond
b) 1.0M KNO3
c) 1.0M C3H8
d) 1.0M C3H5(OH)3
e) 1.0M Cobalt

To answer your question..

there cannot be "1.0 M diamond".. or atleast I haven't read it anywhere.. diamond is insoluble in water (if we think the solvent is water). C3H5(OH)3 ban be soluble a little.. and KNO3 is soluble. C3H8 is not soluble in water. Cobalt is not soluble. It's a Metal! But this question doesn't make sense to me. Your original question (w/o 1.0 M choices) makes sense. And I already provided detailed explanation for that.

---------
Your very first question was in regards to bp definition. -- intermolecular forces and stuff..
your second question was in regards to collegative properties definition.
your third question (1.0 M diamond) doesn't make sense!

 

[Phenotype]

To Choromosome21


Which of the following has the lowest Boiling point?

a) Diamond
b) KNO3
c) C3H8
d) C3H5(OH)3
e) Cobalt

Ans : C

The first and second questions were from Destroyer. Third Question I made up to have better understanding. What I am confused is that why the question above (the first question) answer can't be B using colligative property. I understand colligative property affect bp elevation, mp decrease, and osmotic pressure.

So the question asks which has the LOWEST boiling point, which colligative property does affect isn't it? Sorry if this is still not clear what I'm confused of. -_-;

 

[chromosome21]

alright..i think i understand your question now..

first question: Which of the following has the lowest Boiling point?
second question: Which solution has the lowest freezing point?

These two questions are not similar. the second question is asking for a fp of a "solution".

The first question is asking for bp of a COMPOUND. KNO3 is a compound - ionic compound to be specific. and ionic bonds are very strong.. very hard to break, so we need more energy to break such an ionic bond in order to MELT KNO3.
The second question is asking for fp of a SOLUTION. Solution is made up of solute+solvent. KNO3 is a solute here, and is very soluble in solvent such as water. Colligative properties are for SOLUTIONS, not for COMPOUNDS. Now, when we are boiling a solution, the solvent will want to evaporate, but b/c of the NUMBER of solutes (remember, the 'type' of solutes has nothing to do in colligative properties), the solvent can't evaporate that easily. Resulting the higher bp. In short, solvent alone will be evaporating at a lower bp, but adding the solutes will make the bp go up. Thus, boiling point elevation. Likewise, when a solvent is alone, it'll freeze, say at 0.0 degrees. But adding solute particles will make the solvent to freeze at a lower degree. Thus, freezing point depression. --- remember, we are not MELING KNO3 here, we are not measuring bp of KNO3. We are measuring of a bp of the solvent, and solvent is NOT KNO3. KNO3 is a solute.

Again, Colligative properties are only for SOLUTIONS, not for COMPOUNDS. The first question is for compounds, the second question is for solutions.

If you don't understand, let me know, I will write a detailed explanation for you.

 

[Phenotype]

Thanks a lot ! That answered my question VERY clearly.

Also, your study guide is helping my prep for DAT !