assuming that an aqueous salt solution is distilled at 700 torr .will water boil below ore at or above 100c?

Question

chemistry

Answer 1

Salt will elevate the boiling point and decreased pressure (standard pressure ~750 torr) will depress it. Pure water will boil at 97.7C at 700 torr.

But your problem statement does not have sufficient information. You would need to know the concentration of salt.

I saw a grade school experiment online (see below) where they put salt into water and observed boiling point elevation. They used 2 cups (473g...why don't they teach these kids the Metric System) of water and 2 tbs salt (~50g).

Add 25g salt (~5% solution)...boiling point elevation = 2.7F (1.5C)
Add 50g salt (~10% solution)...boiling point elevation = 5.4F (3.0C)

Therefore, each 1% elevates the boiling point by about 0.3C. Thus I think a solution above 8% would boil above 100C.

Answer 2

If the water is distilled off, then it's pure, no? So pure water would boil at 100C by definition. Sounds like you're distilling a salt solution to make pure water.

But are you trying to boil a salt solution at 700 torr? You need to calculate the bp increase depending on the amount of salt in it, and then calculate the bp depression based on the lower atm pressure of 700 vs the standard 760.

Answer 3

You haven't told us what the concentration of the salt is.
However, the elevation of boiling point caused by the presence of a solute is generally an order smaller than the drop in boiling point caused by a decrease in external pressure.

Unfortunately, I haven't got my data book with me, but I'm fairly confident that the decrease in boiling point caused by a reduction from 760 to 700 Torr will outweigh the increase in boiling point caused by the presence of the salt for all except very concentrated salt solutions. In other words, the salt solution will probably boil at BELOW 100°C