BR chemistry chapter 1 question 26

[vidory]

Hey guys,I'm really confused about about a molar volume question from chapter 1 of BR general chemistry. Here goes...

"...In a second experiment, the researchers places a 5 mL aliquot of the unknown liquid into a capped 1 L flask. The cap has a tiny hole at the top, and the empty flask with cap weighs exactly 120.0 grams. The compound is heated until it reaches a gentle boil. The vapour escapes through the tiny pore in the cap. The liquid continues boiling at 31 degrees C, until none of it remains visible in the flask. The heat source is removed from the flask, and the contents are allowed to cool back to ambient temperature. As the flask cools, the vapour in it condenses into a small pool of liquid at the base of the flask. The flask and cap are then massed with the condensed liquid present. The entire system is found to have a mass of exactly 122.32 grams. That means that the mass of the liquid is 2.32 grams. It is assumed that at the moment when the heat was removed, the flask was completely filled with vapour from the liquid and that all of the air originally in the flask was displaced."

Question: If the organic vapour had not fully displaced all the air from the flask at the time the heat was removed from the flask, how would the results have been affected.

The answer is that the mass of the unknown liquid collected would be too small, so the calculated molecular mass would be too low, and the explanation talks about the actual amount of organic vapour in this case being less than the assumed amount (100%).

But there are so many confusing things about this passage that I don't even know how to start answering the question, let alone in the 1-2 minute interval. Does anyone understand why this is the answer? Also, what is the point to the hole in the top of the flask that is allowing the liquid of interest to evaporate?

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[Odi]

The hole is to allow air to leave the flask and thus be displaced by the vapour.

From the passage you can conclude that they stop the heating once all the initial unknown liquid evaporates and at this point the air displaces completely.

If the heat was removed before the vapour displaces the air completely--not all of the initial liquid would have been evaporated. The condensed "collected" liquid would be less in this scenario.

In the question they state the unknown liquid "collected." Disregard the left over initial liquid that would be present.

It's poorly written IMO. Keep calm and study on.

 

[brood910]

Some of the questions from TBR are really baddd...
Bad not because it's hard, but because it's poorly written + not representative of what AAMC will ask.
But still, the concepts behind these questions are really useful to learn.

 

[inasensegone]

This question is referring to an experiment known as the "Dumas Experiment". It is basically used to determine the molecular mass of an organic volatile liquid. The basic premise is that you will know the volume of the flask (V), the pressure (P), the temperature (T), and the mass of the condensed liquid (m), and therefore you can utilize the ideal gas law (PV = nRT) to determine the molecular mass of the liquid.

How do you utilize the gas law, and why do you have a hole in the top of the flask? Well let's say nearly the whole flask was filled with your unknown liquid. You heat it lightly and allow all of it to evaporate. The organic vapors will displace all of the gas that was originally in the flask (normal air) in addition to some of the organic vapors (actually a lot if you fill up the flask with your unknown) escaping. That's fine though. You don't care that some escapes. You want it to. You stop heating exactly when the last bit of liquid evaporates. So what are you left with? You're left with a flask of known volume that is completely filled with vapor of the organic liquid whose molecular mass you want to determine. Now once all of this vapor re-condenses, because you have removed the heat source, the mass of the liquid is measured (simple weighing process) and this specific amount (mass) has a known volume as a gas. You then re-arrange the ideal gas equation as follows... PV=nRT becomes:

Which then becomes:

OK so what if the unknown liquid didn't displace all of the air that was originally in the flask? This is basically like saying that you didn't add enough of your unknown liquid in the first place. Now you're working with less mass than you need to fill the known volume of your flask. Because M.W. from the re-arranged ideal gas equation is directly proportional to the mass of the condensed liquid, and this mass was too low for that known volume, your calculated M.W. will also be too low.

Edit: This question and passage encompass a few different principles and lines of reasoning that are expected on the MCAT, however I do agree with Brood that this is somewhat poorly worded. Just make sure you're solid on the logic once you get a look at the bigger picture. Speed is definitely important, so looking at a bunch of different problems asking about the same basic science concepts from different perspectives is good because it will help you answer foreign questions more quickly.

Edit 2: Keywords for future searches: TBR The Berkeley Review GChem General Chemistry Chapter 1 page 36 Section 1 Passage 4 (Just because I'm sure other people might find this useful because TBR's explanation bothered me as well)

 

[vidory]

Thanks so much everyone! That was really helpful!!

 

[vidory]

This question is referring to an experiment known as the "Dumas Experiment". It is basically used to determine the molecular mass of an organic volatile liquid. The basic premise is that you will know the volume of the flask (V), the pressure (P), the temperature (T), and the mass of the condensed liquid (m), you can utilize the ideal gas law (PV = nRT) to determine the molecular mass of the liquid.

How do you utilize the gas law, and why do you have a hole in the top of the flask? Well let's say nearly the whole flask was filled with your unknown liquid. You heat it lightly and allow all of it to evaporate. The organic vapors will displace all of the gas that was originally in the flask (normal air) in addition to some of the organic vapors (actually a lot if you fill up the flask with your unknown) escaping. That's fine though. You don't care that some escapes. You want it to. You stop heating exactly when the last bit of liquid evaporates. So what are you left with? You're left with a flask of known volume that is completely filled with vapor of the organic liquid whose molecular mass you want to determine. Now once all of this vapor re-condenses, because you have removed the heat source, the mass of the liquid is measured (simple weighing process) and this specific amount (mass) has a known volume as a gas. You then re-arrange the ideal gas equation as follows... PV=nRT becomes:

View attachment 178710 Which then becomes:
View attachment 178711

OK so what if the unknown liquid didn't displace all of the air that was originally in the flask? This is basically like saying that you didn't add enough of your unknown liquid in the first place. Now you're working with less mass than you need to fill the known volume of your flask. Because M.W. from the re-arranged ideal gas equation is directly proportional to the mass of the condensed liquid, and this mass was too low for that known volume, your calculated M.W. will also be too low.

Edit: This question and passage encompass a few different principles and lines of reasoning that are expected on the MCAT, however I do agree with Brood that this is somewhat poorly worded. Just make sure you're solid on the logic once you get a look at the bigger picture. Speed is definitely important, so looking at a bunch of different problems asking about the same basic science concepts from different perspectives is good because it will help you answer foreign questions more quickly.

Edit 2: Keywords for future searches: TBR The Berkeley Review GChem General Chemistry Chapter 1 page 36 Section 1 Passage 4 (Just because I'm sure other people might find this useful because TBR's explanation bothered me as well)

Would be awesome if BR would hire you to write their explanations lol

 

[inasensegone]

Would be awesome if BR would hire you to write their explanations lol

Haha.. You're welcome, I'm glad it helped. For the most part I think TBR does an awesome job, but just like any other prep company, they will have a couple faults. It's weird because sometimes they have a lengthy explanation for a very basic concept that they have beat like a dead horse, and then they glaze over a more complex concept that could use more clarification. It's frustrating. I'm having such an issue right now with one of their physics problems.

Anyway, good luck!

 

[CookieZine]

This question is referring to an experiment known as the "Dumas Experiment". It is basically used to determine the molecular mass of an organic volatile liquid. The basic premise is that you will know the volume of the flask (V), the pressure (P), the temperature (T), and the mass of the condensed liquid (m), and therefore you can utilize the ideal gas law (PV = nRT) to determine the molecular mass of the liquid.

How do you utilize the gas law, and why do you have a hole in the top of the flask? Well let's say nearly the whole flask was filled with your unknown liquid. You heat it lightly and allow all of it to evaporate. The organic vapors will displace all of the gas that was originally in the flask (normal air) in addition to some of the organic vapors (actually a lot if you fill up the flask with your unknown) escaping. That's fine though. You don't care that some escapes. You want it to. You stop heating exactly when the last bit of liquid evaporates. So what are you left with? You're left with a flask of known volume that is completely filled with vapor of the organic liquid whose molecular mass you want to determine. Now once all of this vapor re-condenses, because you have removed the heat source, the mass of the liquid is measured (simple weighing process) and this specific amount (mass) has a known volume as a gas. You then re-arrange the ideal gas equation as follows... PV=nRT becomes:

View attachment 178710 Which then becomes:
View attachment 178711

OK so what if the unknown liquid didn't displace all of the air that was originally in the flask? This is basically like saying that you didn't add enough of your unknown liquid in the first place. Now you're working with less mass than you need to fill the known volume of your flask. Because M.W. from the re-arranged ideal gas equation is directly proportional to the mass of the condensed liquid, and this mass was too low for that known volume, your calculated M.W. will also be too low.

Edit: This question and passage encompass a few different principles and lines of reasoning that are expected on the MCAT, however I do agree with Brood that this is somewhat poorly worded. Just make sure you're solid on the logic once you get a look at the bigger picture. Speed is definitely important, so looking at a bunch of different problems asking about the same basic science concepts from different perspectives is good because it will help you answer foreign questions more quickly.

Edit 2: Keywords for future searches: TBR The Berkeley Review GChem General Chemistry Chapter 1 page 36 Section 1 Passage 4 (Just because I'm sure other people might find this useful because TBR's explanation bothered me as well)

Thank you so much for the explanation. I was stuck thinking that the question was saying that the mass of the unknown liquid before it was boiled was somehow changed by the presence of air in the flask. It makes so much more sense to say that you just needed more of your unknown liquid to occupy 100% of the volume of the flask. What I am still stuck on is how this relates to the molecular mass. TBR states that if the measured mass of the liquid is too small then the numerator is too small for the calculation of the molecular mass. Is the ideal gas law the only way to make the connection between the mass of the unknown liquid and it's molecular mass?