Firstly I am glad you came back, and thank you for your explanations. It would have been very negative if you had abandoned your first thread here (and I at least would not have been further well disposed, see my signature).
Instead I think this forum can help you significantly improve your study approach. I'd say you need to ask yourself, even allowing for the interruptions, what was it that was stopping you and making you take two weeks even with some help, just to work out as little as the following reasoning?:
"The conductivity should depend on (increasing with) the concentration of ions. In 0.1 M sodium chloride [Na+] and [Cl-] are both 0.1. In 0.1 M phosphoric acid, the ion concentrations must be less than this because part of it is in the associated non-ionic form H3PO4 (concentrations of HPO42- and PO43- are negligible). If ion concentration were the only factor in conductivity we would expect therefore NaCl to conduct better than phosphoric acid - but the fact is the opposite. Therefore ion concentrations alone are not sufficient to explain the given facts, and there must be some other factor in play.”
That is what I meant by reaching first base. (
Not mistaken to look at what the concentrations are - quite necessary)
Many students, and I'm guessing you, have more difficulty learning than necessary because they have a too
passive, not active and questioning, approach. It's partly instilled into them by a teaching problem. Just because passivity has to be combatted, and because it is easy to think you have understood a scientific chapter when you haven't. So it is seen whether you have understood it when you can answer problems and questions on it. Students rarely do this of themselves, so teachers organise the questioning for them. But then there is the problem that the students will feel they are being judged, or graded, with consequences. And perhaps they may be. And this fixes in students’ minds that the only point is to just scrape through answering little questions like this one, and the point of them can thereby be lost.
But
here they are not judged or graded. Or judgement has no consequences. Mistakes don't matter here. That's why I did not alter my initial slight blunder in #4 so as to try and look good. Maybe Borek has made a deliberate mistake in the last post.
And I needed a little time to answer because I had to think about a couple of points mentioned at the end here.
I'd say you needed to give yourself more of a license to think. It was negative that you had this inhibition that you didn't want to think about the further dissociations of the phosphate because that might not have been in the question - that is losing the point of exercises. I would say you would get more benefit from this exercise if you tried to answer:
1 How much actually is the concentration of ions in 0.1 M phosphoric acid? If the dissociation constant is 10
-2 (good enough) I get that the ions are approximately 0.0 27 M. This is not a difficult calculation following on from the definition of dissociation constant.
2 Then for a check and reinforcement try to calculate approximately the concentration of the other ions - look up the dissociation constants. This is an easier calculation - you can make the assumption that they are a small compared to the concentrations of the other things present which simplifies.
Then for the rest of the answer Borek has told you something. But it is something you could have guessed! In proceeding with Socratic method, I would have asked you would you expect the conductivity of even chemically analogous salts, e.g. LiCl and CsBr to be the same? Well to say that this depends on each ion's
mobility is almost obvious, almost by definition, isn't it? At any rate you could make the conjecture. You might even conjecture how mobiities vary according to the salt. And then when you go to your texts or other source of information you might or might not be surprised. If not surprised, that's fine. Surprised, then it will probably register and fix in your mind better than picking it up in passively in lectures. But about the original question, the point is that the mobility of H
+ (and 0H
-) is nearly an order of magnitude greater than that of anything else. Again the active not passive approach – if you had read ahead or even skimmed ahead in your textbook you would have seen this - and why.
The exceptional mobility of H
+ I think is the answer to the question – but wait! What about the phosphate ion, Isn’t that slower and limiting the conductivity? Surprise maybe, the conductivity of the solution is not that of the product of conductivities of the ions or an average - it’s their sum!
Different amounts of current are conducted by the different oppositely charged ions! Not very obvious, at least it wasn't to me, and you will have to read that up in your book. But it's to do with the way that concentrations are in fact changing in a spatially distributed way. In fact measuring these changes is one way to measure the aforementioned ion mobilities – statements about which don't mean anything and is unscientific to talk about for long unless you know how it is measured! Reading or skimming ahead (you might want to skip some of the tedious equations) would give you an idea to think about and would prepare you better for when you hear it in class.
Definitely to think about is:
3 what does the phosphate become when it loses an electron? I needed to think about that I think I've got the answer, haven't found confirmation yet.
I've given several examples, e.g. 1, 2 and 3 of what you would be thinking through with the active approach to learning (any problems with them, come back), and IMHO you would become a more successful student and have a better time if you adopted this approach.
However it could do you no harm to look them up, even if you very validly hope to be able to dispense with them in your explanation, because at the end of the day you want your little bits of knowledge to unite and reinforce each other.
Do they complete? 
