Homework Help: Change in Volume in ethanol

1. Sep 14, 2011

1. The problem statement, all variables and given/known data
Ok, I have a volumetric flask filled with 67ml of ethanol and 36.38ml of water to make up 100ml of solution. My experiment was to find out how temperatue affects volume contraction. Part of my analysis was to justify how much volume contracion changed due to volume, a complicating factor involved was that some of this contracion was due to ethanol and ethanol rather than ethanol water.

The inital temperature was 23 degrees C

3. The attempt at a solution
I started out my mesuring how much the volume of ethanol changed and I calculated that it changed by 4mm and one mm equals 4.7ml, so there was a 0.85ml change in volume.

The temperature had a difference of 40 degrees C so there was difference of 0.02ml/C.

Next I found the percentage of volume change between 23 degrees and 6 degrees which worked out to be 0.125%

= (0.02/17) x 100
approx = 0.125%

I then applied this to the volume of ethanol I put in a flask which was 65ml

= 65 x (o.125/100)
= 0.08 ml/C

The problem is that when the temperature of the flask with the ethanol and water mixture decreases, the volume of ethanol decreases aswell. This means, that the contraction due to ethanol has also decreased. I have taken measurements from 7, 24, 65, 94 degrees and yes, I do have the difference in volumes. All I want are just hints to work out how much contraction is due to ethanol at each of the given temperatures.

Thanks guys, help will be appreciated ;)

2. Sep 14, 2011

Staff: Mentor

Please elaborate, I have troubles comprehending what you are trying to do.

3. Sep 15, 2011

ok this is basically what I did

had a ethanol and water mixture

changed the temperature of the mixture and took note of the changes of volume

filled a test tube with pure ethanol

changed the temperature of the ethanol and took note of the change in volume

Somehow, I have to take away the amount ethanol expanded by from the original water/ethanol mixture. But since I did not have an equal amount of ethanol in both mixtures, I have to do some maths to figure out how much expansion in the original water/ethanol mixture was due to the ethanol itself.

does this help?

4. Sep 15, 2011

Staff: Mentor

Yes. I doubt you can approach it this way. You are assuming interactions to be completely independent, they are not. You can use simple proportions to evaluate how much volume of the ethanol that is present in the mixture with water would expand if it was alone, but assumption that it expands by the same fraction in a homogeneous mixture is IMHO unfounded.

5. Sep 15, 2011

That is exactly what I'm trying to figure out, but I have absolutely no idea of how to approach this though.

6. Sep 15, 2011

Staff: Mentor

Simple proportion:

$$\frac {\Delta V_1}{V_1} = \frac {\Delta V_2}{V_2}$$

should work.

7. Sep 15, 2011

But the thing is that I don't know how much the final volume of the ethanol is? I only know my inital volume which is 67ml (sorry, I made a mistake in my OP)

8. Sep 15, 2011

Staff: Mentor

You've lost me again. You didn't take the second measurement at all?

9. Sep 15, 2011

I took a measurement from the inital volume of the mixture which was 100ml, and I know that mixture's volume rose by 5.205ml. I also know that 67mls of ethanol was in the inital volume. What I'm trying to figure out, is how much of that 5.205 rise, was due to the expansion of ethanol.

10. Sep 15, 2011

Staff: Mentor

But you wrote you did a test measuring change of volume of the pure ethanol - didn't you?

11. Sep 15, 2011

But I didn't use 67 mls of pure ethanol for the pre-test. I didn't even take note of it because there would have been no way for me to get an accurate enough reading using test tubes. Why didn't i use 67mls of ethanol you ask? because I ran out

12. Sep 15, 2011

Staff: Mentor

I am not asking about exact volume you used - have you checked the volume change for ANY amount of ethanol? You stated you did, 0.85 mL was a volume change of some volume of a pure ethanol, wasn't it?

13. Sep 15, 2011

yes, for one of my pre-tests

14. Sep 15, 2011

Staff: Mentor

So you can use ratio to calculate how much volume should change. See my earlier post.

If 1 L changes volume by 1 mL, 0.5 L changes volume by 0.5 mL. See the relation?

15. Sep 15, 2011

yes, but like I've mentioned before... I don't have my inital volume of ethanol for the pre-test. I just have the change over an X amount of degrees.

16. Sep 15, 2011

Staff: Mentor

17. Sep 15, 2011