Boiling Point Equation?

[andywood]

I'm doing a lab for my IB Chemistry class where I'm determining the affect salt content has on the boiling point of tap water. As I was writing out the procedure, I said that I was going to time how long it takes for the water to boil. My question is, given the heat and the time it takes the water to boil, is there an equation to find the boiling point? If not, is there any other way to find it?

[CaptainPanic]

No.

 

The time it takes to heat the water depends on the heat capacity (or specific heat). Also, the amount of energy you need to put into the water depends on the heat capacity.

 

The heat capacity is dependent on the salt concentration though.

 

The boiling point is independent of this, and depends on molecular properties (like for example how polar the molecules are).

 

Finding the boiling point: I advise to look up in handbooks (big books with almost only numbers in tables, and perhaps graphs).

[hermanntrude]

the only things a liquid's boiling point depend on are what the liquid is and the pressure.

 

the only relevant equation i can think of is the clausius-clapeyron equation, which relates the vapour pressure of a liquid at one temperature to the vapour pressure at another. it can be used to find (for instance) the normal boiling point (the boiling point at 1atm). a liquid's boiling point is when the vapour pressure is equal to the external pressure. the only other information you'd need is the heat of vapourisation.

[CaptainPanic]

Ah, yes - That too.

 

At low pressure, the liquid will boil at lower temperature.

On top of a high mountain, water might boil already at 70 deg C... but in a pressure cooker it won't boil even at 140 deg C.

[DJBruce]

You could use the boiling point elevation equation although it would not relate to the time it would take the liquid to boil. You could calculate perform the lab to show the equation is correct.

 

[math] \Delta T_{B}=(k_{B})(m_{s})i[/math]

[Mr Skeptic]

I'm doing a lab for my IB Chemistry class where I'm determining the affect salt content has on the boiling point of tap water. As I was writing out the procedure, I said that I was going to time how long it takes for the water to boil. My question is, given the heat and the time it takes the water to boil, is there an equation to find the boiling point? If not, is there any other way to find it?

 

No, the time it takes to boil depends on the heat capacity, the heat flow, evaporation rate, and the boiling point. The boiling point would be the easiest of these to measure, seeing as all you got to do is put a thermometer in the liquid. You should also read a barometer so you can say what pressure that boiling point is at. The temperature of the liquid will stop rising at the boiling point, as all additional heat added will go to boiling the liquid rather than raising the temperature.

[insane_alien]

if you're just writing up a procedure for the experiment, what's wrong with just sticking a thermometer into it?

[andywood]

Cool, thanks for the help, guys. I'm a bit of a noob at chemistry so I wasn't aware that just sticking a thermometer in it would tell me the boiling point, but now that I think of it, it makes sense. So the concentration of the salt is not going to affect the BP, or did I interpret it wrong? Thanks again for all the help!

[DJBruce]

The concentration of the solute in the solution has the chance to effect the boiling point. This is called boiling point elevation, and is caused by the fact that normally adding a solute lowers the vapor pressure of the solvent. Since the vapor pressure of the solution is lower it must be heated more in order to reach equilibrium with the vapor pressure of the surroundings. The equation that governs this elevation is:

 

[math] \Delta T_{B}=K_{b}m_{sol}i[/math]

 

Where [math] \Delta T[/math] is the change in boiling point.

[math] K_{B}[/math] is the boiling point constant.

[math] m_{sol}[/math] is the molality of th solute.

[math] i[/math] is the Van't Hoff Factor.