Chemistry conceptual question i'm stuck with

[chiddler]

Here we go.

I've been staring at this for the past 20 minutes. I would really like an explanation for the answer.

Here is what I thought, obviously incorrect.

If the vapor has not displaced all the air originally in the flask, then not all the liquid has evaporated. So a good amount of the liquid remains in the flask and therefore the mass of the liquid collected will be too large. Consequently, the calculated molar mass will be too large.

Here is the book explanation which I find a bit confusing.

I know this is a lot of reading, but i'll love you THIS MUCH |........... | more for the help.

Thanks!

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[startswithb]

In the passage it says to assume that all the liquid has evaporated, and in the question stem, that has not changed. The only thing they wish you to consider is that the flask is full of some vapor and some air. Less vapor in the actual flask would mean that when it condenses, the liquid mass measured will be less, therefore the molecular mass (the g of the g/mol) will be too low. The air is not going to condense and add to the mass, so you really just have less of the substance to be measured. I hope this helps!

 

[chiddler]

In the passage it says to assume that all the liquid has evaporated, and in the question stem, that has not changed. The only thing they wish you to consider is that the flask is full of some vapor and some air. Less vapor in the actual flask would mean that when it condenses, the liquid mass measured will be less, therefore the molecular mass (the g of the g/mol) will be too low. The air is not going to condense and add to the mass, so you really just have less of the substance to be measured. I hope this helps!

Thanks for your response! I think i'm misunderstanding what is actually happening. Here it is as I understand it:

Using the assumption: All liquid in flask is heated and vaporized. Then, with all the liquid in gas form, the heat source is removed and whatever gas is present in the container condenses into liquid. Then they measure mass of this liquid.

Without the assumption: The liquid is heated, not all is vaporized. Because the liquid did not all vaporize, a lot of it remained in the flask and therefore the mass would increase.

 

[startswithb]

That makes sense to me. I think the trick is just keeping in mind the assumption.

 

[chiddler]

That makes sense to me. I think the trick is just keeping in mind the assumption.

yeah but that means we get the wrong answer :x

 

[Ssina]

Thanks for your response! I think i'm misunderstanding what is actually happening. Here it is as I understand it:

Using the assumption: All liquid in flask is heated and vaporized. Then, with all the liquid in gas form, the heat source is removed and whatever gas is present in the container condenses into liquid. Then they measure mass of this liquid.

Without the assumption: The liquid is heated, not all is vaporized. Because the liquid did not all vaporize, a lot of it remained in the flask and therefore the mass would increase.

maaan this smells like TBR! well I know so cuz I missed this question also and still can't justify why the answer is D!

correct me if I'm wrong: they put 5ml liquid inside a 1L flask, they head up the liquid until it vaporizes, then they remove the heat source and wait for the vapor to condense back into liquid and finally get the mass of the flask with the liquid that is now condensed back...

they mention that there is the tiny pore for vapor to escape, but I thought it is referring to the air trapped inside, because it also mentions that after heating all liquid vaporizes and fills the flask.

Now how would the mass decrease, if the liquid is vaporized, but doesn't fully replace the air, and then it's condensed and weighted!?

 

[Ssina]

Without the assumption: The liquid is heated, not all is vaporized. Because the liquid did not all vaporize, a lot of it remained in the flask and therefore the mass would increase.

but it says that "as the flask cools, the vapor in it condenses into a small pool of liquid" meaning essentially whatever that was vaporized will condense back adding to the liquid that didn't vaporize... I don't see how that would change the mass?!

 

[MedPR]

Here we go.

I've been staring at this for the past 20 minutes. I would really like an explanation for the answer.

Here is what I thought, obviously incorrect.

If the vapor has not displaced all the air originally in the flask, then not all the liquid has evaporated. So a good amount of the liquid remains in the flask and therefore the mass of the liquid collected will be too large. Consequently, the calculated molar mass will be too large.

Here is the book explanation which I find a bit confusing.

I know this is a lot of reading, but i'll love you THIS MUCH |........... | more for the help.

Thanks!

I just did this passage yesterday, and I remember the question. I haven't read any of the other responses, but here is the answer.

Your logic is wrong because any air left in the flask will not condense into liquid, so it won't contribute to the mass of the liquid. The unknown liquid is made complete of the vapor (not the air). Since the flask has a fixed volume, if there is any air left in the flask, there is no way that you could produce the same amount of liquid as if there was only unknown vapor in the flask. The key here is recognizing the assumption that, when heating stops, the flask is filled with 100% vapor, and 0% air. Going back to the constant volume of the flask, if you have 5% air, you only have 95% vapor. Last time I checked, air does not condense at room temperature, so your condensate (liquid) does not contain any air.

 

[MedPR]

Thanks for your response! I think i'm misunderstanding what is actually happening. Here it is as I understand it:

Using the assumption: All liquid in flask is heated and vaporized. Then, with all the liquid in gas form, the heat source is removed and whatever gas is present in the container condenses into liquid. Then they measure mass of this liquid.

Without the assumption: The liquid is heated, not all is vaporized. Because the liquid did not all vaporize, a lot of it remained in the flask and therefore the mass would increase.

Wrong. In this experiment, there is absolutely no way for the mass of the liquid to increase because at the start there are only 2 things in the flask, the liquid and air. Air does not condense at room temperature.

 

[MedPR]

maaan this smells like TBR! well I know so cuz I missed this question also and still can't justify why the answer is D!

correct me if I'm wrong: they put 5ml liquid inside a 1L flask, they head up the liquid until it vaporizes, then they remove the heat source and wait for the vapor to condense back into liquid and finally get the mass of the flask with the liquid that is now condensed back...

they mention that there is the tiny pore for vapor to escape, but I thought it is referring to the air trapped inside, because it also mentions that after heating all liquid vaporizes and fills the flask.

Now how would the mass decrease, if the liquid is vaporized, but doesn't fully replace the air, and then it's condensed and weighted!?

The question doesn't state that 5mL of the liquid weighs 2.32 grams. They assumed that the flask was completely filled with vapor (1000cc) and because they assumed that the flask was completely full of vapor, they were assuming that whatever mass of liquid they got could be used to calculate the molar mass.

Think of it this way, if I give you a box and tell you how much it weighs when it is empty, and I also tell you it is completely full of some substance, and ask you to find the density of that substance. You'll go ahead and measure the box, calculate the volume of that box, then weigh it and subtract the weight of the box (which I gave you). You are under the assumption that the box is completely full. If I lied to you, and the box wasn't really completely full, your calculated density will be too low because the volume you used was too high.

 

[cheesier]

I just did this passage yesterday, and I remember the question. I haven't read any of the other responses, but here is the answer.

Your logic is wrong because any air left in the flask will not condense into liquid, so it won't contribute to the mass of the liquid. The unknown liquid is made complete of the vapor (not the air). Since the flask has a fixed volume, if there is any air left in the flask, there is no way that you could produce the same amount of liquid as if there was only unknown vapor in the flask. The key here is recognizing the assumption that, when heating stops, the flask is filled with 100% vapor, and 0% air. Going back to the constant volume of the flask, if you have 5% air, you only have 95% vapor. Last time I checked, air does not condense at room temperature, so your condensate (liquid) does not contain any air.

But the question is saying that in one scenario all the liquid turns to vapor and is condensed back down, while in the other some of the liquid turns to vapor while some stays at the bottom of the flask. In both scenarios once the vapors condense the weight should be the same.

What I think the question is implying is that only the condensed vapors are weighed, while whatever was in the aliquot is not.

 

[chiddler]

but it says that "as the flask cools, the vapor in it condenses into a small pool of liquid" meaning essentially whatever that was vaporized will condense back adding to the liquid that didn't vaporize... I don't see how that would change the mass?!

Wrong. In this experiment, there is absolutely no way for the mass of the liquid to increase because at the start there are only 2 things in the flask, the liquid and air. Air does not condense at room temperature.

One of us is misunderstanding the experiment.

5 mL is heated and vaporized. Some of the 5 mL escapes the flask through the hole. So when it recondenses, the liquid minus the amount that escaped reappears. Then they measure its mass.

If not all the liquid is vaporized, then not as much vapor escapes the flask because more of it stays in liquid form. Therefore, it's liquid minus a smaller amount that escaped. They measure the mass to be more than the previous.

So they measure more mass = more molecular weight!

 

[theseeker4]

When heating is halted, some of the original 5mL of vapor has escaped the flask, but the flask is at 1 atmosphere of pressure, is 1L in volume, the temperature is given, so all you need is the constant R to find the mole number n using PV=nRT. Knowing what the mass is, from the weighing after condensation, allows you to determine the molecular weight, since you have n and m.

If some air is in the flask, the volume of vapor is 0.95*1L, not 1L (for example). This makes n smaller than it is assumed to be, so the mass is the mass of 0.95L of vapor when you are assuming it is of 1.0L of vapor. molecular weight = m/n, so the molecular weight is too low.

 

[chiddler]

When heating is halted, some of the original 5mL of vapor has escaped the flask, but the flask is at 1 atmosphere of pressure, is 1L in volume, the temperature is given, so all you need is the constant R to find the mole number n using PV=nRT. Knowing what the mass is, from the weighing after condensation, allows you to determine the molecular weight, since you have n and m.

If some air is in the flask, the volume of vapor is 0.95*1L, not 1L (for example). This makes n smaller than it is assumed to be, so the mass is the mass of 0.95L of vapor when you are assuming it is of 1.0L of vapor. molecular weight = m/n, so the molecular weight is too low.

The liquid is boiled until it is vaporized, then right when the liquid disappears, the heat is turned off. This means that some of the vaporized liquid has escaped the flask, while some of it remains behind and ultimately condenses. This also means that some of the original liquid is removed from the flask.

Their assumption is that what if not all the air is displaced, which means what if not all the liquid evaporates? This means not as much liquid is removed from the flask because less of the liquid was vaporized.

 

[MedPR]

But the question is saying that in one scenario all the liquid turns to vapor and is condensed back down, while in the other some of the liquid turns to vapor while some stays at the bottom of the flask. In both scenarios once the vapors condense the weight should be the same.

What I think the question is implying is that only the condensed vapors are weighed, while whatever was in the aliquot is not.

The question is absolutely not saying that. You are forgetting that some of the vapor could be lost (along with 100% of the air that used to be in the flask). The experiment and molar mass calculation is based on the fact that, when heating stops, you have 1L (or whatever the volume of the flask was, I can't remember) of unknown vapor.

One of us is misunderstanding the experiment.

5 mL is heated and vaporized. Some of the 5 mL escapes the flask through the hole. So when it recondenses, the liquid minus the amount that escaped reappears. Then they measure its mass.

If not all the liquid is vaporized, then not as much vapor escapes the flask because more of it stays in liquid form. Therefore, it's liquid minus a smaller amount that escaped. They measure the mass to be more than the previous.

So they measure more mass = more molecular weight!

1. The passage explicitly states that all 5 mL is vaporized.
2. The calculation of the molecular weight is somehow using the fact that the amount of liquid recondensed results from 1 Liter of vapor. If the calculation wasn't using that value (1L of vapor), then there would be no reason to do the experiment.

Why are you speculating ("So they measure more mass = more molecular weight!") on a situation that is the opposite of what is explicitly stated in the passage?

 

[leoni101]

Help me understand. Mol mass = mass/mols. Mass is measured, mols is calculated from measured volume. 22.41 Liters in one mol of gas. If you measure 1 liter of gas/22.41 = .0446 mols of gas. 2.32g/.0446 mol of gas = 52.018 mol mass

Compare this with lets assume the hypothetical situation the book says, not all of it is vapor, some is air. Then we have .95L gas/22.41 = .0424 mol of gas. 2.32g/.0424 mol of gas = 54.717. Doesn't this mean that your calculated mol mass is too high?

 

[theseeker4]

The liquid is boiled until it is vaporized, then right when the liquid disappears, the heat is turned off. This means that some of the vaporized liquid has escaped the flask, while some of it remains behind and ultimately condenses. This also means that some of the original liquid is removed from the flask.

Their assumption is that what if not all the air is displaced, which means what if not all the liquid evaporates? This means not as much liquid is removed from the flask because less of the liquid was vaporized.

No, it means that air and vapor are both in the flask. The fact that the liquid was all boiled away did not change. Nowhere does it say that liquid remained in the flask.

 

[leoni101]

Help me understand. Mol mass = mass/mols. Mass is measured, mols is calculated from measured volume. 22.41 Liters in one mol of gas. If you measure 1 liter of gas/22.41 = .0446 mols of gas. 2.32g/.0446 mol of gas = 52.018 mol mass

Compare this with lets assume the hypothetical situation the book says, not all of it is vapor, some is air. Then we have .95L gas/22.41 = .0424 mol of gas. 2.32g/.0424 mol of gas = 54.717. Doesn't this mean that your calculated mol mass is too high?

Oooh never mind I get it. It's saying that if you calculated situation 1 when situation 2 actually happened then situation 1 compared to situation 2 is too low. Is this the right thought process?

Even if this is right, the answers don't say anything about the volume, it only says about the collected mass. Your mass is observed by measurement, not calculated, so the problem in the calculation isn't mass, it's volume.

 

[theseeker4]

Oooh never mind I get it. It's saying that if you calculated situation 1 when situation 2 actually happened then situation 1 compared to situation 2 is too low. Is this the right thought process?

Exactly. The 1L assumption stands, which means you think you have n when you really have 0.95n and therefore, your molar mass is too low.

 

[leoni101]

Exactly. The 1L assumption stands, which means you think you have n when you really have 0.95n and therefore, your molar mass is too low.

But can you explain why the answer says the mass is too low instead of volume too low? The mass was measured, not calculated, so how can it be too low?

 

[theseeker4]

Even if this is right, the answers don't say anything about the volume, it only says about the collected mass. Your mass is observed by measurement, not calculated, so the problem in the calculation isn't mass, it's volume.

It is conceptual, you have to understand what exactly you are calculating, and how, to determine what will happen if one of the assumptions is wrong. The mass is what it is, but the difference between assumed n and actual n is what makes the calculated molar mass lower.

 

[cheesier]

The question is absolutely not saying that. You are forgetting that some of the vapor could be lost (along with 100% of the air that used to be in the flask). The experiment and molar mass calculation is based on the fact that, when heating stops, you have 1L (or whatever the volume of the flask was, I can't remember) of unknown vapor.

I didn't mean to imply that all of the vapor was collected, just that all of the liquid was vaporized.


Edit: answer makes sense now

 

[theseeker4]

But can you explain why the answer says the mass is too low instead of volume too low? The mass was measured, not calculated, so how can it be too low?

It is too small for 1L of vapor at 1 atm of pressure at the given T. Since we are assuming 1L, when in fact it is 0.95L, the mass measured is too low for 1L.

 

[MedPR]

Help me understand. Mol mass = mass/mols. Mass is measured, mols is calculated from measured volume. 22.41 Liters in one mol of gas. If you measure 1 liter of gas/22.41 = .0446 mols of gas. 2.32g/.0446 mol of gas = 52.018 mol mass

Compare this with lets assume the hypothetical situation the book says, not all of it is vapor, some is air. Then we have .95L gas/22.41 = .0424 mol of gas. 2.32g/.0424 mol of gas = 54.717. Doesn't this mean that your calculated mol mass is too high?

Your calculations are right, but your understanding is wrong. The passage says to assume 1 L of vapor and 0L of air.

Oooh never mind I get it. It's saying that if you calculated situation 1 when situation 2 actually happened then situation 1 compared to situation 2 is too low. Is this the right thought process?

Even if this is right, the answers don't say anything about the volume, it only says about the collected mass. Your mass is observed by measurement, not calculated, so the problem in the calculation isn't mass, it's volume.

Wrong again. You're right that the passage doesn't explicitly state that volume is important, but if the volume of gas didn't matter, there would be no reason to vaporize anything. Just measure weigh the 5mL of unknown liquid and be done. It should be obvious that the volume of gas condensed into mass of liquid is important simply from the fact that the experiment is being conducted. Your collected mass is observed by measurement, your MOLAR MASS is calculated.

Do you know the difference between mass and molar mass?

 

[MedPR]

If all of the liquid is vaporized and the air is not displaced, wouldn't more liquid be collected??? How does that mesh with the given answer?

If AIR condensed at ROOM TEMPERATURE, there would be no air for you to breathe right now. You would be drowning in liquid.

 

[leoni101]

Your calculations are right, but your understanding is wrong. The passage says to assume 1 L of vapor and 0L of air.



Wrong again. You're right that the passage doesn't explicitly state that volume is important, but if the volume of gas didn't matter, there would be no reason to vaporize anything. Just measure weigh the 5mL of unknown liquid and be done. It should be obvious that the volume of gas condensed into mass of liquid is important simply from the fact that the experiment is being conducted. Your collected mass is observed by measurement, your MOLAR MASS is calculated.

Do you know the difference between mass and molar mass?

Hey MEDPR, there is a difference between trying to help someone and being a troll. If you get annoyed helping somoene then don't bother responding.

The Answer specifically states: "The mass of unknown liquid collected would be too small, so the calculated molecular mass would be too low." The second part is true, but the first part is not. The mass is what it is, it was measured, not calculated. Obviously I know what molar mass is if my calculations were right.

 

[theseeker4]

If all of the liquid is vaporized and the air is not displaced, wouldn't more liquid be collected??? How does that mesh with the given answer?

If all of the liquid is vaporized, it has to go somewhere. Whether the air is all displaced or there is air remaining, the small hole in the cap allows some quantity of the vapor to escape. The pressure will be 1atm whether there is air in the flask or not, so there will be more molecules of the compound in the flask if the air is entirely displaced than there would be if there is air remaining. (Since PV=nRT, T is given, P is constant due to the hole in the cap, and V is given and constant)

When the compound is condensed back to liquid, only the quantity of molecules that were in the flask at the point heating was halted will turn to liquid. If there is no air in the flask, you will have more liquid than you would if some of the volume of the flask were taken up with air, because you had more molecules of the vapor. That is why the mass is lower if the flask had air in it still than it would be if the air was all displaced.

 

[theseeker4]

Hey MEDPR, there is a difference between trying to help someone and being a troll. If you get annoyed helping somoene then don't bother responding.

The Answer specifically states: "The mass of unknown liquid collected would be too small, so the calculated molecular mass would be too low." The second part is true, but the first part is not. The mass is what it is, it was measured, not calculated. Obviously I know what molar mass is if my calculations were right.

The mass is too small, for 1L of vapor. Yes, the mass is technically correctly observed, meaning that exact amount of liquid is collected, but it is too low for 1L of vapor, which is what they mean by the mass being collected is too small. The explanation could be worded better, it took me a bit to figure out why D was correct, but for the assumed 1L of vapor, the mass is, in fact, too low.

 

[leoni101]

Your calculations are right, but your understanding is wrong. The passage says to assume 1 L of vapor and 0L of air.



Wrong again. You're right that the passage doesn't explicitly state that volume is important, but if the volume of gas didn't matter, there would be no reason to vaporize anything. Just measure weigh the 5mL of unknown liquid and be done. It should be obvious that the volume of gas condensed into mass of liquid is important simply from the fact that the experiment is being conducted. Your collected mass is observed by measurement, your MOLAR MASS is calculated.

Do you know the difference between mass and molar mass?

Not saying that volume doesn't matter. Please read carefully instead of assuming the person you're talking to is a *****. I'm saying the answer is incorrectly saying your mass is too low. The mass is measured. A more correct statement for the first part would be to say that the calculated mols is incorrect due to lower volume, which will make your molar mass too low.

 

[leoni101]

The mass is too small, for 1L of vapor. Yes, the mass is technically correctly observed, meaning that exact amount of liquid is collected, but it is too low for 1L of vapor, which is what they mean by the mass being collected is too small. The explanation could be worded better, it took me a bit to figure out why D was correct, but for the assumed 1L of vapor, the mass is, in fact, too low.

Thanks, I appreciate your response. Got it.

 

[chiddler]

ok i understand it now! thanks!

MedPR, i really appreciate your help but please try to be more respectful in your writing.

 

[MedPR]

Not saying that volume doesn't matter. Please read carefully instead of assuming the person you're talking to is a *****. I'm saying the answer is incorrectly saying your mass is too low. The mass is measured. A more correct statement for the first part would be to say that the calculated mols is incorrect due to lower volume, which will make your molar mass too low.

No, the mass would be too low for 1L of vapor. .5L of vapor does not condense to the same mass as 1L of the same vapor.

Hey MEDPR, there is a difference between trying to help someone and being a troll. If you get annoyed helping somoene then don't bother responding.

The Answer specifically states: "The mass of unknown liquid collected would be too small, so the calculated molecular mass would be too low." The second part is true, but the first part is not. The mass is what it is, it was measured, not calculated. Obviously I know what molar mass is if my calculations were right.

Again, the mass would be too low. I guess you don't know what a troll is either? If you think I'm being condescending or mean about how I'm answering, that's fine, but that's not the same as trolling. Just FYI.

A troll would be someone who explains why and convinces you that completely wrong answer is true.

 

[MedPR]

Aside from what is explicitly stated in the passage, the only concept to recognize for this question is that air does not condense to liquid at room temperature. If you keep that in mind, you won't get the question wrong.