Isnt molality independent of of temperature?

[brood910]

I've read that molarity changes with temperature but molality does not as only volume is affected by the temperature so many times from different scientific journals.

But, apparently, TBR does not think so.
Chem Ch. 3 Passage #8 Q56:

"Which solution has the highest B.P?"
I know that MX must have higher B.P. than MY as it has lower molecular mass.
Their answer key says "The solution with the highest B.P is the soln with the highest molality of impurities (DUH...). Because the density of water decreases as the T of water increases, the mass of 10ml water is greater at lower T. This means that largest molality is found with 10 ml water at higher T because mass solvent is least (?????????).

Ok, so they are saying the mass solvent DECREASED because the density decreased at higher T. How the hell is this possible? I understand that they reasoned this out because the given volume of the solvent is constant, but isnt this question COMPLETELY wrong? Density is affected by the T because the VOLUME changes.

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[QuinnTheEskimo]

I've read that molarity changes with temperature but molality does not as only volume is affected by the temperature so many times from different scientific journals.

But, apparently, TBR does not think so.
Chem Ch. 3 Passage #8 Q56:

"Which solution has the highest B.P?"
I know that MX must have higher B.P. than MY as it has lower molecular mass.
Their answer key says "The solution with the highest B.P is the soln with the highest molality of impurities (DUH...). Because the density of water decreases as the T of water increases, the mass of 10ml water is greater at lower T. This means that largest molality is found with 10 ml water at higher T because mass solvent is least (?????????).

Ok, so they are saying the mass solvent DECREASED because the density decreased at higher T. How the hell is this possible? I understand that they reasoned this out because the given volume of the solvent is constant, but isnt this question COMPLETELY wrong? Density is affected by the T because the VOLUME changes.

Molality is defined as moles solute/kg solvent.

If you are given samples of 10 ml of water at different temperatures, the highest temperature will have the LEAST dense H2O, and therefore the LEAST mass of H2O.

therefore, molality is highest at the higher temperature.

 

[brood910]

You did not understand my question there.
It is impossible to change the mass of the solvent simply by changing the temperature, unless it is high enough to evaporate it. Like I said above, density changes because VOLUME changes, not mass. Yes, mathematically, in this situation, because the volume is constant, mass should change, but that's stupid as it is logically impossible.

If the situation in this question is scientifically possible, then all those scientific journals I've read are wrong as they clearly said molality is independent of temperature.

 

[QuinnTheEskimo]

You did not understand my question there.
It is impossible to change the mass of the solvent simply by changing the temperature, unless it is high enough to evaporate it. Like I said above, density changes because VOLUME changes, not mass. Yes, mathematically, in this situation, because the volume is constant, mass should change, but that's stupid as it is logically impossible.

I can't see the question, but are you sure you are reading the experimental steps correctly?

Say you are given 1 mole of a substance.

You have a 30 degree sample of water and a 90 degree sample of water. Both are 10 ml. You put 0.5 a mole in one test tube, and 0.5 moles in the other test tube. The 90 degree sample is going to have higher molality.

You are correct that if you then take this 90 degree sample and cool or heat it, the molality will not change, as mass of solvent will not change.

But I can't really answer the specifics without seeing the question.

 

[brood910]

lol that's not true at all. There isnt any difference between the two situations you provided, with respect to molality.
You should think about how density is affected by the temperature.

 

[QuinnTheEskimo]

lol that's not true at all.
You should think about how density is affected by the temperature.

huh? what's not true? As I said above, the density decreases as temperature increases.

Therefore, a 90 degree sample of 10 ml water WILL weigh less than a 30 degree sample of water at the same volume (10 ml).

 

[brood910]

-_- I am repeating this for the 4th time now..
Density changes because of VOLUME, not mass. Mass does not change at all..
Yes, MATHEMATICALLY, what you said is true with a constant volume, but like I said, SCIENTIFICALLY, that's not gonna happen as volume changes as temperature changes. Read what other people say more carefully, dude..

 

[QuinnTheEskimo]

-_- I am repeating this for the 4th time now..
Density changes because of VOLUME, not mass. Mass does not change at all..
Yes, MATHEMATICALLY, what you said is true with a constant volume, but like I said, SCIENTIFICALLY, that's not gonna happen as volume changes as temperature changes. Read what other people say more carefully, dude..

Ok, now you are being really rude, and I am trying to help you.

As I said before, I think your misunderstanding is in the experimental steps.

I can't see the question, but I do not believe they are talking about heating and cooling a single sample of water (in that case, mass of solvent would not be variable).

I will repeat again, think about this experimental example:

You are given 1 mole of a substance (call it X).

You have a vial of 10 ml of water at 30 degrees.

You have a vial of 10 ml of water at 90 degrees.

You dissolve 0.5 moles of X in the 30 degree water.

You dissolve 0.5 moles off X in the 90 degree water.

The 90 degree sample will have higher molality. Do we agree on this point?

 

[brood910]

Yes, I do agree with that MATHEMATICALLY, but not SCIENTIFICALLY as volume is NEVER constant in RL when temperature changes. That's why I made this post in the first place to point at the question maker as it is impossible to happen in RL.

MCAT is all about critical thinking about something that CAN happen in RL. This question is stupid as volume cannot be constant when temperature changes. Do you understand my point?

 

[QuinnTheEskimo]

Yes, I do agree with that MATHEMATICALLY, but not SCIENTIFICALLY as volume is NEVER constant in RL when temperature changes. That's why I made this post in the first place to point at the question maker as it is impossible to happen in RL.

Ok, but EXPERIMENTALLY, you can measure 10 ml of hot water and 10 ml of cold water! You can have 2 different water samples at different temperatures! It does not have to be the SAME sample that is heated and cooled. You get me?

 

[QuinnTheEskimo]

Yes, I do agree with that MATHEMATICALLY, but not SCIENTIFICALLY as volume is NEVER constant in RL when temperature changes. That's why I made this post in the first place to point at the question maker as it is impossible to happen in RL.

MCAT is all about critical thinking about something that CAN happen in RL. This question is stupid as volume cannot be constant when temperature changes. Do you understand my point?

No, I do not understand your point at all, actually.

I *think* what you are saying is that if you have a vial of water, and you heat it, the volume changes. Yes, that is true.

What I am saying is that you can measure a specific VOLUME (10 ml) of water at 2 different temperatures. In that case, you have 2 different masses of water. Mass is the variable there, not volume. the volume is the same in each sample.

 

[brood910]

It does not matter if they are two different samples. You are using the same amount of volume + same amount of moles. In real life, MASS never changes. So, molality WONT change, but molarity changes. You keep saying that different samples will give different results than when you heat/cool one sample when in fact they are the same.

 

[QuinnTheEskimo]

It does not matter if they are two different samples. You are using the same amount of volume + same amount of moles. In real life, MASS never changes. So, molality WONT change, but molarity changes. You keep saying that different samples will give different results than when you heat/cool one sample when in fact they are the same.

Milliliters is a unit of volume.

10 ml of hot water is LESS DENSE than 10 ml of cold water. This means that, by definition of density, the 10 ml of hot water WEIGHS LESS.

If you dissolve a substance in the hot water and the cold water, the hot water solution will have higher molality.

 

[brood910]

OHHH you are saying that those samples are prepared AFTER heating/cooling?

 

[QuinnTheEskimo]

OHHH you are saying that those samples are prepared AFTER heating/cooling?

I am saying that the samples are prepared at different temperatures, but the SAME VOLUME (10 ml).

So yes, after heating and cooling

 

[Odi]

No dudez you're doin it wrong... When you heat the water it increases global warming and then Angelina Jolie adopts another baby from a third world country making D the correct answer.




You explained it well from the get go Quinn +1

 

[Lifeman]

deleted

 

[drechie]

so..... molality doesn't change with temperature? (my EK Bio 1001 book said it did circa 2003).

 

[BerkReviewTeach]

so..... molality doesn't change with temperature? (my EK Bio 1001 book said it did circa 2003).

A given solution will have a constant molality with varying temperature, as long as solvent is not added or removed.

However, as Quinn explained very well (and patiently), this question was not about the same solution undergoing a change in temperature. This question compared different solutions made at different temperatures. What Brood repeatedly missed is that you were given 10 mL (a volume, not a mass) of pure water at different temperatures. As the temperature goes down, the pure water becomes denser, so 10 mL will be more and more massive.

Adding a set amount of solute to 10 mL of hot water will have a greater (mole solute)/(mass solvent) value than when adding a set amount of solute to 10 mL of cold water, because the denominator is bigger for the solution made using cold water.

 

[chemist16]

To re-emphasize the key point for the OP, nobody is heating or cooling anything. NO heating or cooling is involved. You have in front of you two separate and independent graduated cylinders with water and they are at different temperatures.