# Melting point and solubility in water of benzoic acid

#### jnimagine

in literature, find melting point and solubility in water of benzoic acid, vanillin, phthalic acid, salicyclic acids. Calculate the expected volume of hot water that would be requried to dissolve 0.15g of each compound.

With info on melting point and solubility in water... how can you predict the amount of volume of water needed?? I know that you start by adding one drop and if it dissolves you add 10 drops more and if it doesn't dissolve you add 0.25ml more until it does dissolve... but how do you know how much to add if you're only given the melting point and the solubility?!?!?

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#### Borek

Mentor
Re: recrystallization?!

Solubility should be enough, not sure what melting point has to do with the question.

#### GCT

Homework Helper
Re: recrystallization?!

in literature, find melting point and solubility in water of benzoic acid, vanillin, phthalic acid, salicyclic acids. Calculate the expected volume of hot water that would be requried to dissolve 0.15g of each compound.

With info on melting point and solubility in water... how can you predict the amount of volume of water needed?? I know that you start by adding one drop and if it dissolves you add 10 drops more and if it doesn't dissolve you add 0.25ml more until it does dissolve... but how do you know how much to add if you're only given the melting point and the solubility?!?!?

Is this for Physical Chemistry? There are equations that are available to use with your given parameters , however this seems to be for an organic chemistry lab.

#### jnimagine

Re: recrystallization?!

Is this for Physical Chemistry? There are equations that are available to use with your given parameters , however this seems to be for an organic chemistry lab.
yes this is for organic chem lab...
and.. I'm not getting it... :(

#### Borek

Mentor
Re: recrystallization?!

OK, completely forgot. GCT referred to this equation:

$$\ln x = -\frac {\Delta H_m} R(\frac 1 T - \frac 1 T_m)$$

This is so called (at least in Polish) ideal solution approximation to the solubility. x is a molar fraction of solute, T is temperature, Tm is a melting point, delta Hm is enthalpy of melting. In your case enthalpy of melting divided by R is just a constant:

$$\ln x = C (\frac 1 T - \frac 1 T_m)$$

Knowing solubility in some temperature T and melting point you can calculate value of this constant for your substance, that in turns gives you simple equation linking solubility with temperature.

Problem is, this equation is as inaccurate as it is simple. It is only an approximation. For some solutions it works nicely, for some it fails. And in reality it is not really helpfull in the lab, as it doesn't tell anything about solubility of the substance in different solvents - it suggests that solubility of the substance in all solvents is identical, which is clearly wrong. That's why in most cases it is more effective to check solubilities in different solvents and different temperatures experimentally.

#### GCT

Homework Helper
Re: recrystallization?!

Yeah that seems to be the right equation , however , he or she mentioned that this was for an organic lab. jnimagine , have you covered this equation? Even if you did not , you may use it to impress your teachers.

#### jnimagine

Re: recrystallization?!

Yeah that seems to be the right equation , however , he or she mentioned that this was for an organic lab. jnimagine , have you covered this equation? Even if you did not , you may use it to impress your teachers.
noo... we haven't covered any of that... since it's organic.
hmm... those equations will make me look smart for sure :P
but I don't think they want to see calculations... more a theoretical interpretation...

#### Borek

Mentor
Re: recrystallization?!

Strong interactions between molecules in crystal -> high melting temperature & low solubility (unless interactions between molecules of solvet and solute are very strong, but that'll be no longer ideal solution).

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