Chad question...possible mistake

[Cofo]

---

Members do not see this ad.

If the vapor pressure of pure water at 20°C is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?

This is the only information Chad gives in the question. Isn't this question impossible to solve without Chad giving us what the mol fraction of water is?

In the explanation, Chad says 5.5m CH3OH has 5.5 moles CH3OH for every 1kg H2O

...how am I supposed to know that?

 

[FROGGBUSTER]

that's just the definition of molality.

molality = moles solutes/kg solvent.

i think you can solve it if you know the definition of molality.

 

[Strag]

If the vapor pressure of pure water at 20°C is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?

This is the only information Chad gives in the question. Isn't this question impossible to solve without Chad giving us what the mol fraction of water is?

In the explanation, Chad says 5.5m CH3OH has 5.5 moles CH3OH for every 1kg H2O

...how am I supposed to know that?

hey just did this yesterday.

what the other poster said. you're supposed to convert 5.5 moles CH3OH for every 1 kg H2O into mol H2O. You can do this through stoichiometric conversions kg H2O to g H2O to how many g H2O.

1 kg H2O x 1000 g H2O x mol H2O
1 kg H2O 18 g H2O

Should be around 50-55 ish.

you set this up with your mol fraction of mol H2O/total mol (so whatever you find for mol H2O + 5.5) then times that by 18 which is the pure pressure.

This will give you the partial pressure for water.

 

[Cofo]

hey just did this yesterday.

what the other poster said. you're supposed to convert 5.5 moles CH3OH for every 1 kg H2O into mol H2O.

How the hell am I supposed to know to do this though? The question doesn't say how many kg of H20 there is. And I don't remember Chad ever explaining it like this. This can't be right...the question does not even say the amount of total solution. Please, can someone else explain this? Thank you strag, but I'm just not understanding it...🙁

In Chad's example during the video...he actually gives us Pure H20 compared to a container with 9 mol H20 and 1 mol CH3OH. Why doesn't he do this for the question that I'm having trouble with? ...This S#IT must be a terribly worded question!!!

 

[FROGGBUSTER]

How the hell am I supposed to know to do this though? The question doesn't say how many kg of H20 there is. And I don't remember Chad ever explaining it like this. This can't be right...the question does not even say the amount of total solution. Please, can someone else explain this? Thank you strag, but I'm just not understanding it...🙁

In Chad's example during the video...he actually gives us Pure H20 compared to a container with 9 mol H20 and 1 mol CH3OH. Why doesn't he do this for the question that I'm having trouble with? ...This S#IT must be a terribly worded question!!!

When you are given molality, you know the mol fraction.

The molality is 5.5 mols CH3OH/1kg H2O. The mol fraction is 0.1. This is going to be the same no matter your mass of water. If you don't believe me, you can go ahead and do calculations.

It's a perfectly good question...

 

[seunglim87]

How the hell am I supposed to know to do this though? The question doesn't say how many kg of H20 there is. And I don't remember Chad ever explaining it like this. This can't be right...the question does not even say the amount of total solution. Please, can someone else explain this? Thank you strag, but I'm just not understanding it...🙁

In Chad's example during the video...he actually gives us Pure H20 compared to a container with 9 mol H20 and 1 mol CH3OH. Why doesn't he do this for the question that I'm having trouble with? ...This S#IT must be a terribly worded question!!!

If the vapor pressure of pure water at 20°C is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?

This is the only information Chad gives in the question. Isn't this question impossible to solve without Chad giving us what the mol fraction of water is?

In the explanation, Chad says 5.5m CH3OH has 5.5 moles CH3OH for every 1kg H2O

...how am I supposed to know that?

before you go apesh@t about a question and doubt the ability of the questionmaker, you should understand the inadequate capability of your brain. there is nothing wrong with the question.

This question is basically testing on your knoweldge of vapor pressure, utilizing application of mole fraction and understanding of molality.

molality= mole of solute/ 1kg of solvent

Vapor pressure of water in nonpure solution= (mole fraction of water in the solution) x (vapor pressure of pure water)

water is used in this example, but any substance can be used

5.5m = 5.5 moles of ch3oh + 1 kg of water(= (1000g of water)/(18g/mol of water) = 55.5 moles of water) = > 61 moles total in the solution

vapor pressure of water in nonpure solution= (55.5/61)x(18 torr) = 16.37 torr should be the answer

 

[UCSD1984]

before you go apesh@t about a question and doubt the ability of the questionmaker, you should understand the inadequate capability of your brain. there is nothing wrong with the question.

This question is basically testing on your knoweldge of vapor pressure, utilizing application of mole fraction and understanding of molality.

molality= mole of solute/ 1kg of solvent

Vapor pressure of water in nonpure solution= (mole fraction of water in the solution) x (vapor pressure of pure water)

water is used in this example, but any substance can be used

5.5m = 5.5 moles of ch3oh + 1 kg of water(= (1000g of water)/(18g/mol of water) = 55.5 moles of water) = > 61 moles total in the solution

vapor pressure of water in nonpure solution= (55.5/61)x(18 torr) = 16.37 torr should be the answer

omg it's Chad

 

[Strag]

How the hell am I supposed to know to do this though? The question doesn't say how many kg of H20 there is. And I don't remember Chad ever explaining it like this. This can't be right...the question does not even say the amount of total solution. Please, can someone else explain this? Thank you strag, but I'm just not understanding it...🙁

In Chad's example during the video...he actually gives us Pure H20 compared to a container with 9 mol H20 and 1 mol CH3OH. Why doesn't he do this for the question that I'm having trouble with? ...This S#IT must be a terribly worded question!!!

So the definition of molality is the moles of solute/kg of solvent.

It says you have 5.5 m of something right? That means for every 5.5 mols of CH3OH, you have 1 kg of H2O which is your solvent.

I think it's the definition that is keeping you from understanding where your kg solvent is coming from...me and another poster already set up the stoichiometric calculations for you so you should be able to go from there. I believe Chad also laid out the calculations too.

This one stumped me a couple times as well, but just make sure you understand it and you'll be golden 👍.

 

[seunglim87]

omg it's Chad

no i am a chad worshiper LOL

 

[chance8]

Looks like answer is the same as above;
Molality = mole of compound/ kg of solvent
The solvent is H20

1. Find the total # of moles for compound CH3OH +H2O.
Given: 5.5 molality of CH3OH. This is 5.5 mole of CH3OH per 1 kg of H2O.
Find mole of H2O, given 1 mole H2O = 18 g
1 kg of H2O = 1000 g
1000 g H2O (1mole H20/18 g H2O) = 5.56 mole H2O
Add: 5.5 mole CH3OH + 5.56 mole H2P = 61.06 mole total

2. Find mole fraction of solvent: 55.56/61.96=0.91
3. Find Vapor pressureof water above the solution: Vapor pressure is pressure of pure solvent multiplied by mole fraction of solvent = 18 torr x 0.91=16.4 torr

 

[Cofo]

...you should understand the inadequate capability of your brain. there is nothing wrong with the question.

I was studying for 12 hours yesterday, so I was on the brink of losing my sanity and I was frustrated, but I understand the concept now. Your pejorative remark did not offend me because I know I'm intelligent. But I hope you don't continue being disparaging like this when other SDN members inquire help. A few of the material/concepts will be more difficult for others to understand...this does not equate a person to having an "inadequate brain".
If you continue to use SDN, I really hope you will have more understanding of how challenging studying for the DAT is. A lot of the material, people haven't covered in 4+ years. Grow up, and treat people with more respect and lose the superiority complex. Don't put others down when they are trying to better oneself.

 

[FROGGBUSTER]

I was studying for 12 hours yesterday, so I was on the brink of losing my sanity and I was frustrated, but I understand the concept now. Your pejorative remark did not offend me because I know I'm intelligent. But I hope you don't continue being disparaging like this when other SDN members inquire help. A few of the material/concepts will be more difficult for others to understand...this does not equate a person to having an "inadequate brain".
If you continue to use SDN, I really hope you will have more understanding of how challenging studying for the DAT is. A lot of the material, people haven't covered in 4+ years. Grow up, and treat people with more respect and lose the superiority complex. Don't put others down when they are trying to better oneself.

It was because you were too quick to label Chad's question as "****" without acknowledging the possibility that you just didn't understand it. If you had chosen your words more carefully, I doubt seunglim would have made fun of you. You came off as full of yourself. Truth be told, the concept you were struggling with is just a very simple matter of stoichiometry that no one who's taken General Chem before should be struggling with.

Anyways, next time choose your words more wisely and the rest of us will be glad to help you.

 

[Cofo]

It was because you were too quick to label Chad's question as "****" without acknowledging the possibility that you just didn't understand it. If you had chosen your words more carefully, I doubt seunglim would have made fun of you. You came off as full of yourself. Truth be told, the concept you were struggling with is just a very simple matter of stoichiometry that no one who's taken General Chem before should be struggling with.

Anyways, next time choose your words more wisely and the rest of us will be glad to help you.

I haven't taken gen chem is over 4 years. That's a damn long time. I understood what molality was, but to me, the question was awkwardly phrased and I thought important information was missing. I wasn't the only person who was confused by a very simple matter of stoichiometry (read above...)
And I definitely didn't come off as pretentious. I admitted I was having trouble understanding a concept and because I thought important info was missing from the question, I labeled the question as a possible bad question (it's even in the title, dude). *yawn* this is getting so inane. peace, and thanks frog for your help earlier, it was appreciated.

 

[seunglim87]

I haven't taken gen chem is over 4 years. That's a damn long time. I understood what molality was, but to me, the question was awkwardly phrased and I thought important information was missing. I wasn't the only person who was confused by a very simple matter of stoichiometry (read above...)
And I definitely didn't come off as pretentious. I admitted I was having trouble understanding a concept and because I thought important info was missing from the question, I labeled the question as a possible bad question (it's even in the title, dude). *yawn* this is getting so inane. peace, and thanks frog for your help earlier, it was appreciated.

Hey cofo, i am sorry if i hurt your feeling🙁 i apologize using word like "inadequate brain", i am sure you are a very intelligent person (for real). i know how frustrating it can be when you are studying for DAT and get stuck on something. Its just that i have so much respect for chad because he helped me tremendously to get through DAT. I did all his questions in one sitting the day before my DAT, and i dont think there was a single mistake in the questions, in my opinion. whenever you have trouble with something(especially with gchem) i will try to help out even though i dont come on SDN as much as i used to before my DAT. good luck on your DAT buddy. i am sure you will do well;i know so because you care