General Chemistry 2 quiz question -- Help, anybody?

[tietjen]

I missed this lab.... and I hate chemistry. I really do. This is the last chemistry class I need. Anyways, can anyone help? What is the expected freezing temperature of a solution of 2.00 g of CCl4 in 100 g of benzene, C6H6? (For benzene, m.p. = 5.5oC, Kf = 5.12oC kg/mol) A. 4.8oC B. 5.8oC C. 3.2oC D. 8.5oC Not a clue what to do...

I know its a little late but heres the tutorial:

First find the moles of CCl4 in a 2g:
Molecular Weight of CCl4 = 154g
-so-
2.00g x (1mol/154g) = .0141 mol CCl4

-Now-
change in T=KMi

change in T=(5.12)(.0141 mol CCl4/.1 kg sol'n)

change in T= 0.722

FP of Benzene is 5.5 - 0.722 = 4.8

The answer is A

[JoshuaLogan]

 

I know its a little late but heres the tutorial: First find the moles of CCl4 in a 2g: Molecular Weight of CCl4 = 154g -so- 2.00g x (1mol/154g) = .0141 mol CCl4 -Now- change in T=KMi change in T=(5.12)(.0141 mol CCl4/.1 kg sol'n) change in T= 0.722 FP of Benzene is 5.5 - 0.722 = 4.8 The answer is A

 

 

 

Thanks. That .1 kg comes from the given 100g, right?

 

Because molality = moles / kg ?

[tietjen]

Thanks. That .1 kg comes from the given 100g, right? Because molality = moles / kg ?

 

 

Exactly. That threw me for about 1 hour the other day... I kept forgetting to put things into kg and wondered why I was always off by a decimal.

 

Ya Chem sucks... the only true constant is just that fact... Chem sucks.

 

To bad its the layout of all things in the universe

[JoshuaLogan]

Exactly. That threw me for about 1 hour the other day... I kept forgetting to put things into kg and wondered why I was always off by a decimal. Ya Chem sucks... the only true constant is just that fact... Chem sucks. To bad its the layout of all things in the universe

Yeah, I think I'd actually like organic chemistry! lol no equations to remember...

I actually find some parts of chemistry pretty interesting... but that might be because of my interest in psychedelic drugs

hahaha

[tietjen]

Yeah, I think I'd actually like organic chemistry! lol no equations to remember... I actually find some parts of chemistry pretty interesting... but that might be because of my interest in psychedelic drugs hahaha

Ya I'm sure if you work hard you could be a great "chemist" if thats what we want to call it... but if you don't work hard you'll just end up blown up in a meth lab.

[JBecker]

Chemistry is awesome.

 

And yeah, multiple choice can always be solved like knucklefux. I haven't done solution chemistry like that since my first semester (I'm a junior chem major), but understanding the basics of thermodynamics/colligative properties + a sense of scale is all you need for that one.

[JBecker]

In fact, first thing that comes up on Google when you search the word "colligative" is http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html#c1

[JoshuaLogan]

Ya I'm sure if you work hard you could be a great "chemist" if thats what we want to call it... but if you don't work hard you'll just end up blown up in a meth lab.

 

 

lol noo I would never try that. I just find some of the stuff interesting. like how some of the psychedelics work on the brain because of their chemical shape and such

[JBecker]

lol noo I would never try that. I just find some of the stuff interesting. like how some of the psychedelics work on the brain because of their chemical shape and such

You're mostly talking biochem now :cop:

Though real chemists can understand the nitty-gritty parts of that kind of chemistry better than most biochemists can. That's why I work in a chemical biology lab.

[JoshuaLogan]

 

Chemistry is awesome. And yeah, multiple choice can always be solved like knucklefux. I haven't done solution chemistry like that since my first semester (I'm a junior chem major), but understanding the basics of thermodynamics/colligative properties + a sense of scale is all you need for that one.

 

 

Yeah, I understood the colligative property.... the freezing temp dropping. It's liek the same reason why they put salt on roads... so they freeze and ice over at a lower temperature than normal.

 

I just wasn't sure if it was 4.8 or 3.2... seemed like it could've possibly been either without working it out. I gusesed 4.8 because of it being closer to the original temps. I got lucky haha

[JBecker]

 

Yeah, I understood the colligative property.... the freezing temp dropping. It's liek the same reason why they put salt on roads... so they freeze and ice over at a lower temperature than normal. I just wasn't sure if it was 4.8 or 3.2... seemed like it could've possibly been either without working it out. I gusesed 4.8 because of it being closer to the original temps. I got lucky haha

 

 

It's kind of like a refrigerator in a room. Yes, the temperature of the room has to go up to get the temperature inside the refrigerator to go down. But because we're often talking about something like 100x the space, often you're talking about a change of <.1 or so degree k in the room for drop needed fridge.>

[JBecker]

Yeah, I understood the colligative property.... the freezing temp dropping. It's liek the same reason why they put salt on roads... so they freeze and ice over at a lower temperature than normal. I just wasn't sure if it was 4.8 or 3.2... seemed like it could've possibly been either without working it out. I gusesed 4.8 because of it being closer to the original temps. I got lucky haha

 

 

I'm pretty sure the explanation, btw, is that some of the energy that's "removed" from the solution is instead removed from solute and not just solvent.

 

At least, I'm pretty sure for boiling point it's the presence of solute along with on the surface that accounts for the difference, so I imagine the explanation for freezing is similar.