Re: The boiling point of salt water is not constant. Why?

John,

You are correct in thinking that the boiling point should be raised by further addition of salt. If you know the molecular weight of the solute (sodium chloride in this case) you can calculate the ebullioscopic constant for water. Once this constant is known, you can determine the molecular weight of a different soluble compound.

M = k(w/eW)

So what went wrong?

• It is normal to use a type of apparatus designed by Beckmann (very early part of this century)in which boiling solvent is returned to the heated flask by means of a condenser. Be sure that cool condensate is not running down the thermometer causing an artificially low reading. Question: how can you compensate for the weight of (pure) water in the condenser?
• From the calculation given above, be sure that all weights are properly accounted for.
• Be sure that the temperature has fully stabilized after the introduction of fresh solute.
• Did you correct for ambient atmospheric pressure changes? Is it possible that you conducted the experiment over a significant time span? At least long enough for atmospheric pressure to drop (such as may be caused by stormy weather) so as to cause the boiling point to decrease? Remember that the boiling point of water is 100 degrees celcius only at one standard atmosphere (760 mmHg).
• From the values that you gave, your boiling point elevations are high so I can only assume that the amount of water was quite small in relation. It is possible that with vigorous boiling salt was deposited above the surface of the boiling water thereby actually reducing its concentration in the water.
• Take care with your experiment and make sure that you are not mistaking the evolution of dissolved air for true boiling.

Good luck!
Martin Thomas