Determing Boiling Point elevations in lab

[mheadlee]

I am trying to get the calculations and the data to match up in lab. The boiling point elevation predicted by the equation does not match up to the one I am getting by direct measurement. What am I doing wrong?

 

[Borek]

I am trying to get the calculations and the data to match up in lab. The boiling point elevation predicted by the equation does not match up to the one I am getting by direct measurement. What am I doing wrong?

We don't know what equation you are talking about. We don't know what you did. We don't know what and how you calculated. We don't know what and how you measured. We know nothing, yet you expect us to give you a definitive answer? Sorry, we are witty, wise and knowledgeable, but mind reading is something we are not trained in.

 

[mheadlee]

I am trying to determine the Boiling point elevation for a 1 m solution of sodium carbonate. Multiplying 1 m by .512 should give a boiling point elevation of .512 C, but when I measure out a 1 m solution and boil it I get an elevation of 2 C

 

[Borek]

What is the formula for boiling point elevation for ionic substances?

 

[Dr_Morbius]

I believe the molar boiling point elevation for ionic compounds depends on the number of ions that are produced when the substance ionizes. Sodium Carbonate ionizes to two Sodium plus one Carbonate that makes 3 particles therefore a 1 molar solution of Sodium Carbonate should have 3 moles of ions in it.

http://en.wikipedia.org/wiki/Boiling-point_elevation

 

[mheadlee]

My calculations were based on 1 mol. That explains why the elevation I am getting is between 3 and 4 times higher. The rest of the error can be attributed to the equipment I am working with. Its like trying to slice an onion with a chain saw. Thanks for your help.

 

[Borek]

What you were missing is called a Van 't Hoff factor.

 

[mheadlee]

Thanks again - We tried it and cut our error down from about 300% to about 40%. Much better but not good enough. I wonder if it would be better to work backwards from an observed boiling point to a Van't Hoff factor? I need to "brush up" on this stuff a little more.

 

[Borek]

How precise was the thermometer used to measure the BP elevation?

 

[mheadlee]

we were using digital thermometers plus minus .1 C

 

[Borek]

That means your delta T in worst case is ±.2 and very likely ±.1 deg C. If you did a single experiment you can't say anything about accuracy of your measurements (other than to assume it was in the range given by the instrument specification).

 

[mheadlee]

We did three trials, all three outcomes were within 10% of each other. Our lowest error was 40% and our highest error was 50%. I am wondering if the actual Van't Hoff value is somewhere between one and three. Three assumes all ions dissociate completely and one assumes that none did. Also - I wonder if the concentration of the solute was too high. Would the actual VH value drop as the solution approaches saturation? (I am guessing at this point)

 

[Borek]

Three assumes all ions dissociate completely

As far as I know this is a pretty good assumption in this case.