Solubility Curves

[gonelli]

I was wondering if anyone knew if there is a formula or equation that can be used to calculate certain variables when using solubility curves? I know that graphs can be used to oberve the curve and work out approximatly what the temperature or solubility is at a certain point.

 

In particular I wanted to know if there is a way to work out or find an equation for calculating the solubility curve for potassium nitrate dissolved in water.

 

Any help would be great.

[iLove]

hmm there is a formula...

m/v*100 gives u the concentration

remember that the mass is an independent variable (doesnt change)

as u know tht x axis is temp and y is concentration

 

for example..if we are using 3g of mass then the equation will be

3/2*100 = 150g at certain temp

 

hope it helps

[hermanntrude]

I think you're confused.

 

The formula you used was for the mass/volume percentage, which is just one measure of concentration. The normal concentration unit to use is molarity, which is the moles of solute divided by litres of solution.

 

You also applied your formula wrongly. The "m" is the mass of the solute. the "v" is the volume of solvent. An example would be if we dissolved 10g of NaCl in enough water to make a 100mL solution. m/v% would then be 10g/100mL *100% =10%. This measure of concentration is an unusual one usually only used in pharmaceuticals.

 

As for the original question, i'm afraid i'm not sure if there's an equation.

[John Cuthber]

In general there isn't a nice well defined equation, but for potassium nitrate in water I think a plot of the log of the solubility vs temperature would be aproximately a straight line. There are a few solubility curves here.

http://www.saskschools.ca/curr_content/chem30_05/4_solutions/solution3_1.htm

 

Things get really complicated if you have something like sodium sulphate which has different hydrated forms.

http://en.wikipedia.org/wiki/Sodium_sulfate

[CaptainPanic]

John Cuthber put you on the right track. You'll have to find experimental data (that means you need a book that gives you the numbers... and the people who wrote that book got it from experiments).

 

Then with a list of points (different solubility at a different temperature) you can use a fit-program to make a line through it. The most simple, and (imho) one of the worst, programs is Excel. It has the "trendline" to fit the data.