Question

Two-hundred grams of phenol (C6H5OH) are dissolved in 1.5 L of water. At what temperature does this solution freeze?

Answer 1

Delta T_b = i * K_b * m

You need K_b ( the boiling elevation constant for water, according to Purdue University it's 0.512 degree C per mole)

Then you need to convert the 200g phenol into moles (find the molar mass and divide 200 by the molar mass)

The van hoff factor for phenol is questionable because it can deprotonate but it's a weak acid so it doesn't deprotonate completely. So the van't hoff factor is between 1 and 2. If you have to take a whole number then I would take 1.

There you go, you have i, k_b, and m, now solve for delta T_b and add 100 to it, that's your answer

Answer 2

Um, when you are looking for freezing wouldn't you take delta T_b and subtract it from 0?

Answer 3

@alias

Answer 4

oh shoot this is freezing. So sorry!

The formula will be the same. The constant in this case would be different, and yes you substract it from 0.

Good catch!

Answer 5

Ok, just checking! Give me a minute, I'll be back with the answer, and you can check it.

Answer 6

Hey @alias, how do I solve for i? Sorry I'm such a noob! lol

Answer 7

and m?

Answer 8

yea, like i said i should be a value between 1 and 2 because it's an acid. That number is closer to 1 since it's not a strong acid. So pick 1

m depends on the units of your constant. Is it degree C per mol? then m should the the mol of phenol. In that case convert 200 grams into mol by using the molar mass

Answer 9

Oh, ok, so I found out that there are 2.12 moles in the 200 gram sample. Then judging by your instructions above, I figured out that K_b= 1.08. So I'm going to ignore the van hoff factor, just because it hasn't been mentioned in class, and take my 1.08 and multiply it by 2.12, equalling 2.29. Does this seem right?

Answer 10

@alias

Answer 11

where did you get the 1.08 for the constant?

Answer 12

Well, I took the .512 and multiplied it by 2.12, which equals 1.08?

Answer 13

because according to different uni websites it's 1.86 C/mol, so...

Unless your teacher gave you that number...

Answer 14

I'm so lost rn

Answer 15

sorry... I completely wasted your time right now. the m is molality, not mols, so it's unit is kg/L

Answer 16

So do we start over?

Answer 17

been a while since i did this. Ok... so here we go again:

i = 1
K = 1.86, it's a constant and you get this number in your textbook (should be in a table o something. I got this number from chem literature)
m = molality. 200g is 0.2kg right? So divide 0.2kg by 1.5L and that's your molality

Answer 18

It's completely fine, too, I'm going to be up for a long time.

Answer 19

Ok, so molality is .133333333

Answer 20

yea, so then multiply 0.13 * 1 * 1.86. take 0 and minus that number, that's your answer. It should be negative

Answer 21

Ok, soo, I get .13*1*1.86=.242
Then, 0-.242=-.242 seems right?
Thanks for the persistence throug my stupidity!

Answer 22

that's right!

And no, I'm sorry! It's cuz I messed up in the beginning and confused you

Answer 23

ppsh, whatever. It's okay. Help is help, and I'm not looking a gift horse in the mouth.