[luvana]

Hi,

I'm doing a lab report on determining the solubility of Ca(OH)2 by titration (with HCl in a burette and Ca(OH)2 with indicator in a flask).
So far, this is the data I've fetched:

V(Ca(OH)2 = 10 cm^3
V(HCl) = 8.23 ml
c(HCl) = 0.05 mol dm^3

What I have done is firstly finding the number of moles of HCl and Ca(OH)2 with the equation: n = (concentration*volume)/1000. Afterwards, I used the formula "concentration = n/V" to determine the concentration of Ca(OH)2.

This was a method I saw on a not-so-reliable web page. Apparently, the resulted concentration of Ca(OH)2 is also the molar solubility. What I'm wondering is if this is true? Because it will mean that concentration is the same as molar solubility.

What I did afterwards was to use the formula Ksp = [Ca2+][OH-]^2 to determine the product solubility.

My second question is if anyone can come up with more uncertainties?
So far I have the uncertainty of the burette, pipette and temperature. I know that I have to calculate the percentage uncertainty as well.

In short, my questions are:
- Is concentration the same as molar solubility? If not, how do I find the molar solubility from this experiment?
- Aside from the ones I mentioned, are there any other significant uncertainties I haven't thought of?
- Is it correct to calculate the percentage uncertainty of HCl in this experiment? Or do I calculate the uncertainty of Ca(OH)2? Or both?

Thanks!

[Borek]

the resulted concentration of Ca(OH)2 is also the molar solubility. What I'm wondering is if this is true? Because it will mean that concentration is the same as molar solubility.

How are both things defined?

What I did afterwards was to use the formula Ksp = [Ca2+][OH-]^2 to determine the product solubility.

No, that's not solubility. That's solubility product.

- Is it correct to calculate the percentage uncertainty of HCl in this experiment?

Where did you got the HCl concentration from? Was it determined by you, or were you just given solution of a known concentration?

[luvana]

the resulted concentration of Ca(OH)2 is also the molar solubility. What I'm wondering is if this is true? Because it will mean that concentration is the same as molar solubility.

How are both things defined?

What I did afterwards was to use the formula Ksp = [Ca2+][OH-]^2 to determine the product solubility.

No, that's not solubility. That's solubility product.

- Is it correct to calculate the percentage uncertainty of HCl in this experiment?

Where did you got the HCl concentration from? Was it determined by you, or were you just given solution of a known concentration?

Thanks for replying!

1. I know that concentration and molar solubility are different by definition, which is why I was skeptical. However, they are closely linked. Is this a wrong method of finding the molar solubility then? Could you suggest another way?

2. Then how do I calculate the solubility of Ca(OH)2, if that is not how it is done? Could you maybe direct me to a web page with this information?

3. I was given a solution with a known concentration.

I haven't done many lab reports, so I'm completely clueless about this.

[Borek]

I know that concentration and molar solubility are different by definition, which is why I was skeptical. However, they are closely linked.

List both definitions.

Then how do I calculate the solubility of Ca(OH)2, if that is not how it is done?

List the definition of solubility. You are mistaking solubility with a solubility product here, but you won't move forward without clearly defining them.

3. I was given a solution with a known concentration.

Then (unless you were given information about its uncertainty) you have no choice but to assume it is an exact number.

[luvana]

I know that concentration and molar solubility are different by definition, which is why I was skeptical. However, they are closely linked.

List both definitions.

Then how do I calculate the solubility of Ca(OH)2, if that is not how it is done?

List the definition of solubility. You are mistaking solubility with a solubility product here, but you won't move forward without clearly defining them.

3. I was given a solution with a known concentration.

Then (unless you were given information about its uncertainty) you have no choice but to assume it is an exact number.

Concentration: is the amount of solute in a known volume of solution.
Molar solubility: is the number of moles of solute that can be dissolved in one liter of solvent to saturate the solution.
Solubility: the maximum quantity of a substance that may be dissolved in another.

[Borek]

Concentration: is the amount of solute in a known volume of solution.

Close, but slightly wrong. While you are right about ratio, what does it mean "1 M" solution? Or, to put it differently: what "known" volume of the solution?

Molar solubility: is the number of moles of solute that can be dissolved in one liter of solvent to saturate the solution.

OK

Solubility: the maximum quantity of a substance that may be dissolved in another.

Once again - you are right about the general idea, but in fact - if you look at units - solubility can be expressed in a more rigorous way. This is exactly the same problem as with the definition of the concentration.

[luvana]

Concentration is essentially a measure of the number of particles in a solution. It can be expressed as the number of moles in a particular volume.

Solubility is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium.

While I appreciate your time and help, it'd be nice with something more specific so I can keep working on my report which is due tomorrow. Thank you!

[magician4]

some additional information you might find usefull:

What I did afterwards was to use the formula Ksp = [Ca2+][OH-]^2 to determine the product solubility.

asides from the what Borek already mentioned, I'd like to point out that this approach will lead to questionable results  (as you will see when you compare the Ksp gained hereby to the one known to literature , Ksp 5.02 * 10^-6 mol³/L³ )

reason is , that most (sic !) of the calcium in a saturated solution of calciumhydroxide is not present as "Ca²⁺" , i.e. [Ca(H₂O)₆]²⁺
instead, it "hides" in other complexes ( [Ca(H₂O)₅(OH)]⁺ , [Ca(H₂O)₄(OH)₂] , to name them), as calciumhydroxide is not a strong base (i.e. completely dissociated) under these conditions*⁾, and those complexes are not being accounted for in the Ksp known to literature!  (only the "real" [Ca(H₂O)₆]²⁺ is)

  that's one of the major reasons , why your result will differ from the Ksp known to literature

regards


Ingo



*⁾
the base constants of Ca(OH)₂ are pKb 1 = 1,37 and pKb2 = 2,43 , respectively
... and this will lead to kind of buffers at pH ~ 12.75 (that's what a saturated calciumhydroxide solution usually shows) , which then will cause the problems in recalculation mentioned above

[Borek]

Concentration is essentially a measure of the number of particles in a solution. It can be expressed as the number of moles in a particular volume.

That's almost word for word what you wrote earlier. I asked: what it means "1 M" solution?

While I appreciate your time and help, it'd be nice with something more specific so I can keep working on my report which is due tomorrow. Thank you!

Please answer my question as asked instead of repeating the same answers again and again. We would be much closer to the end now. I am trying to guide you to the understanding, but it requires cooperation.