[Glorzifen]

Thanks in advance to anyone who takes the time to peruse the questions below:

1. Precipitation Reactions

"What is the impact of dissolving PbI2 in a 1.0M sol'n of Pb(NO3)2?"

I'm not sure I'm understanding how to do these questions. I have been trying to use the solubility guidelines (1-7, in my textbook) to understand this but can't get the right answers consistently.

The options for each question are:
(a) Increase solubility?
(b) Decrease solubility?
(c) No effect no the equilibrium

Am I right in thinking...'Well if PbI2 is added to Pb(NO3)2...then since initially Pb(NO3)2 by itself was soluble but now it forms products, 1 of them soluble/aqueous and 1 of them insoluble...the solubility decreases?'

But what about: "What is the impact of adding PbI2 to a 1.0M sol'n of NaCl?", according to the notes; the solubility increases.

I think I'm missing some (or a lot) of the intuition behind this.

2. Solutes as Electrolytes:

Which is the strongest conductor?
(a) CaCl2(aq,1M)
(b) CH3CH2OH(l)
(c) CH3COOH(aq,1M)
(d) HCl(aq,1M)
(e) H2O(l)

Is this the same as asking 'Which is the strongest acid?' (because stronger acids dissociate more readily...because of weaker bonds). I reasoned this out based the electronegativity difference between CaCl2 vs. HCl...figuring that the higher the difference, the weaker the bond would be and therefore the more it would dissociate and conduct. Is that the wrong way to think about it? What about the bases? And what does the bracketed stuff have to do with this (aq vs liquid and the 1M)?

3. Solubility Product for Salts:

What is the molar solubility of chromium(II) hydroxide (Ksp=2.0E-16)?
Cr(OH)2 -> Cr+ + 2OH-
2.0E-16 = (x)(2x)^2
2.0E-16 = 4x^3
x = 3.7E-6 (WRONG according to the notes...the correct answer is: 4.6E-6. What did I miss?)

What is the molar solubility of lead(II) azide (Ksp=2.5E-9)?
Pb(N3)2 -> Pb2+ + 2N3-
2.5E-9 = (x)(2x)^2
2.5E-9 = 4x^3
x = 8.5E-4 (This is CORRECT according to the notes. Why is this one right and the first one wrong?)

I look forward to learning some chemistry.

Thanks again,

G

[Borek]

"What is the impact of dissolving PbI2 in a 1.0M sol'n of Pb(NO3)2?"

Poor wording if you ask me - impact on what? Probably on the salt solubility?

Am I right in thinking...'Well if PbI2 is added to Pb(NO3)2...then since initially Pb(NO3)2 by itself was soluble but now it forms products, 1 of them soluble/aqueous and 1 of them insoluble...the solubility decreases?'

What 'products' does it form? Take a look just at K_so, or think in terms of Le Chatelier's principle or common ion.

But what about: "What is the impact of adding PbI2 to a 1.0M sol'n of NaCl?", according to the notes; the solubility increases.

Honestly - I am not sure about this one. Some complexing by chlorides and increased ionic strength will increase solubility, no doubt about it, but I am not sure if these effects ar already between those you are supposed to know.

Is this the same as asking 'Which is the strongest acid?'

No. What does conduct electricity in the liquids?

What is the molar solubility of chromium(II) hydroxide (Ksp=2.0E-16)?
Cr(OH)2 -> Cr+ + 2OH-
2.0E-16 = (x)(2x)^2
2.0E-16 = 4x^3
x = 3.7E-6 (WRONG according to the notes...the correct answer is: 4.6E-6. What did I miss?)

Perhaps water autodissociation?

[408]

What is the molar solubility of chromium(II) hydroxide (Ksp=2.0E-16)?
Cr(OH)2 -> Cr+ + 2OH-
2.0E-16 = (x)(2x)^2
2.0E-16 = 4x^3
x = 3.7E-6 (WRONG according to the notes...the correct answer is: 4.6E-6. What did I miss?)

Why is this one right and the first one wrong?)

look how low the Ksp is.  Look at how low the hydroxide concentration generated is.  You must consider the dissociation of water to OH- and H3O+ in this case

[Glorzifen]

Okay. I'm vaguely following...but how do I go about taking that into account? My x is the [OH-], right? Does some of it become H2O or something?

[Borek]

On the second thought - something is wrong. While for sure OH^- from water autodissociation have to be taken into account (it may not play substantial role, but 10^-7 is not far from concentrations you are getting, so it is worth a try before deciding if it should be ignored), its presence should lower solubility, yet your notes state that the solubility is higher. Again, as in the case of the lead iodide solubility - this is not an impossible result, but a lotdepend on the model used for calculations.

And general approach to calculations in such a case should be to take all equilibria into account (that is, Kw and Kso), write all mass balances, charge balance - and solve, checking what approximations can be made.

[Glorzifen]

I'm figuring out some of this stuff.

With respect to my 2nd question concerning the strongest conductor etc...it depends on to what extent the said compounds ionize; correct? Strong acids/bases and their neutral salts ionize most readily (strong electrolytes) while weak acids/bases and their conjugates ionize to a lesser extent (weak electrolytes) and then there are non-electrolytes...such as CH3CH2OH. Now, some follow up questions concerning this example:

Which is the strongest conductor?
(a) CaCl2(aq,1M)
(b) CH3CH2OH(l)
(c) CH3COOH(aq,1M)
(d) HCl(aq,1M)
(e) H2O(l)

(a) is a neutral salt of a strong acid and a strong base (Ca(OH)2 and HCl), so its a strong electrolyte/conductor. (d) is a strong acid though...so how do I know that CaCl2 dissociates more readily than HCl? Does it come down the 2 Cl atoms in CaCl2 which are more electronegative than the 1 Cl in HCl (which would create a greater different in electron density thus making Ca easier to dissociate than H)?

Also, how do I identify non-electrolytes? How is it that I'm supposed to look at CH3CH2COOH and know that it won't dissociate?

Finally what does being liquid vs. aqueous have to do with it?

Thanks again, I'm learning a lot.

[Borek]

With respect to my 2nd question concerning the strongest conductor etc...it depends on to what extent the said compounds ionize; correct?

Yes & no. Yes - degree of ioznization matters. No - that's not the only thing that matters. The more dissolved ions, the higher their charges, the better conductivity (that's not always true, as each ion has its own characteristics, but that's a good starting point).

Now think, how do HCl and CaCl₂ compare?

Also, how do I identify non-electrolytes? How is it that I'm supposed to look at CH3CH2COOH and know that it won't dissociate?

CH₃CH₂COOH seems to be a carboxylic acid, and as such it weakly dissociates. However, there is no such compound on the list. There are CH₃CH₂OH - ethanol, and CH₃COOH - acetic acid. These you just should remember.

Finally what does being liquid vs. aqueous have to do with it?

One is a pure substance, other is solution in water.

[Glorzifen]

Now think, how do HCl and CaCl₂ compare?

Ah...CaCl₂ makes a stronger electrolyte because there are more atoms present (CaCl_2*)?

[Borek]

Not only more atoms, but Ca²⁺ carries twice the charge. That doesn't have to mean it will be better at conduction than H⁺, as H⁺ is much more mobile (thus it can cover much larger distance in the same time), but as first approximation the higher the charge, the better.