[zmasterflex]

We just did a lab where we distilled a Toluene (bp 110C) and Dichloromethane (bp 40C) mixture. The average bp value is 75C but the dichloromethane boiled out (and condensed) at an average of 49 degrees. The problem is, is that when we distilled/boiled out the remaining toluene the temperature reading was approximately 90C which is 20 degrees lower than its standard bp. Thermometer was placed between round bottom flask and west condenser. All joints sealed tightly. Not under vacuum. Any help would be appreciated. Thanks

[fledarmus]

There are a number of possibilities:

1) Did you calibrate your thermometer? And are you absolutely sure there were no breaks in the mercury column? (assuming a mercury thermometer, of course)

2) Was the bulb of your thermometer entirely immersed in the vapor that you were attempting to measure a temperature for? A change in position of 5mm is enough to make the reading completely unreliable.

3) Was the toluene collected pure? You can measure the boiling point of the collected product - if the boiling point of the product is also 90°C, then your setup was correct and your toluene is impure. If the boiling point of your product is 110°C, then either the thermometer position or the thermometer itself is wrong.

[zmasterflex]

Thanks for the response, Im sorry for posting this in wrong forum its organic chemistry. Everyone in the lab (all 20 of us) got the same wrong number it's probably due to one of the factors you mentioned, most notably purity. thanks