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Solubility and Precipitation of Unknown Ion

  1. Jan 2, 2015 #1
    1. The problem statement, all variables and given/known data

    A colorless solution is known to contain one of these
    ions. Which ion is present if adding dilute HCl produces
    a white precipitate that dissolves when the solution is
    warmed?
    (A) Ag+
    (B) Cu2+
    (C) Hg22+
    (D) Pb2+

    2. Relevant equations
    N/A

    3. The attempt at a solution
    I know it isn't B, as copper ion solution is not colorless. I can somewhat rule out A, as silver chloride is incredibly insoluble relative to the other options. I'm a bit lost after this however. Possible answers remain A, C, and D.

    Thanks in advance.
     
  2. jcsd
  3. Jan 2, 2015 #2

    Bystander

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    What else do you know about solubilities?
     
  4. Jan 3, 2015 #3
    There are no Ksps given. I'm not sure what you mean by what else I know :/
     
  5. Jan 3, 2015 #4

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    There has been material presented in class, or in the text book, or in the lab prep. notes, hasn't there?
     
  6. Jan 3, 2015 #5
    This is from the 1999 USNCO exam. I've already completed class. I feel like I am missing something very obvious, I'm just not sure what exactly I'm missing.
     
  7. Jan 3, 2015 #6

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    The differences in solubilities of Pb+2 and Hg2+2 chlorides is not obvious. It's something to be looked up in handbooks, picked up in prep. notes for qualitative analysis labs, or just memorized. There might be some basis for a "hand-waving" argument that Hg2+2 is so large that there is little hydration energy available to drive a solution process, therefore making it only slightly soluble, or that PbCl2 is a more conventional ionizable solute, therefore more soluble and increasingly soluble at higher temperatures, but none of this is obvious.
     
  8. Jan 4, 2015 #7

    Borek

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    Of all weakly soluble chlorides here only one dissolves when warmed. This is often listed as its important property in solubility rules.
     
  9. Jan 4, 2015 #8

    epenguin

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    (I tried to post this 9h ago but some site glitch prevented me.)

    Your answers A and B are fine and as reasoned as one can. There is no way that I know that you can know which of the two chlorides is soluble on warming without looking it up, being told, or best, being shown, or doing it yourself.

    It is just a vague memory for me, you have brought back something had not gone through my mind for many decades. But we used to have to know it at school - we did 'qualitative analysis' and had to identify the elements or groups present in a mixture, and if I remember the way to distinguish plumbous and mercuric was exactly by the redissolving when heated. Though I didn't really remember which that was! OK it's Hg2+. But check. If I remember, from the same class demonstrations and lab excercises, mercuric chloride is not really white - It's slighty yellowish, a bit like cream. Don't take my word for that either! If that is true I can tie it to the use of a form of PbCl2 in white paint (but that is also many decades ago!). I wish I could tie the solubility facts into something else like that to help remember, perhaps someone else can.
     
    Last edited: Jan 4, 2015
  10. Jan 4, 2015 #9

    Borek

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    OK, to make things clear - solubility of PbCl2 at 0°C is 0.67 g/100 g of water, and 3.34 g/100 g of water at the boiling point. Fivefold difference. If you precipitate some solid chloride and it dissolves on heating, it is definitely PbCl2.
     
  11. Jan 4, 2015 #10

    epenguin

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    Oh dear, this is difficult. The Solubility of nearlyneverything increases with temperature, and that of PbCl2 increases as much or more.

    "Its solubility increases from 6% at 20 °C (68 °F) to 36% in 100 °C (212 °F)." http://en.m.wikipedia.org/wiki/Mercury(II)_chloride
    Maybe what you'd be seeing is another unpredictable fact:
    "In the presence of chloride ions, it dissolves to give the tetrahedral coordination complex [HgCl4]2−. "
     
  12. Jan 4, 2015 #11

    Borek

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    But the question is not about HgCl2.
     
  13. Jan 4, 2015 #12

    epenguin

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    I took Hg22+ to be a typo as I was unaware there is such a thing, is there? and that it would be in a high-school syllabus.
    But I checked it out and find there is :blushing:. I think we used to call it HgCl. Like in another recent question we now know that what we used to call P2O5 (and often still do) is really P4O10, though it doesn't matter for most purposes.

    Whatever we call mercurous chloride it is not quite unimportant, because of its use in calomel electrodes.
     
    Last edited: Jan 4, 2015
  14. Jan 6, 2015 #13
    Sorry I'm so late in replying. Thank you all for your posts! Guess I just need to memorize some more MSDSs! :-p
     
  15. Jan 18, 2015 #14

    epenguin

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    I also apologise for time before I could come back.
    As you recognise you have a not very easy part of your course to get on top of.
    I'd suggest you not postpone this and that you do it progressively so you get on top rather than it being a fearsome barrier looming on top of you.
    Trouble is it is of course not deducible like math or a lot of physics (so as was said there was no way you could have worked out those solubilities). It is however somewhat connected so you should try and learn connecting everything, using mind maps. Every new thing try to connect it to the others making a progressively expanding mind map. For every new thing you return to the old, test own knowledge by active recall method, connect new thing. Don't be too limited by whether you have had any particular thing in class yet.

    If you have not been able to see certain things in the laboratory look for them on YouTube.

    Some things to put on your map


    Here two links about lead chloride

    http://www.chemguide.co.uk/inorganic/group4/lead.html PbCl2-



    They do not mention the redissolution by excess Cl-
    There is lots of detail in the wiki article on it, and solubility of forms of lead sulphate has to do with battery life you will see. But more important than detail is its broad principle, see next link. Unlike any other battery I know it plays with three levels of oxidation of the metal. Electrons are withdrawn from Pb metal oxidising it to to form PbSO4

    A few Connections to make are to

    'qualitative analysis'.

    Solubility rules and tendencies and exceptions (on which qualitative analysis depends)

    Paints and pigments

    'calomel electrode'.

    And soon to 'lead-acid battery' which is not obvious and is harder than others to learn and is probably on your syllabus.

    There is lots of detail in the wiki article on it, and solubility of forms of lead sulphate has to do with battery life you will see. But more important than detail is its broad principle, see next link, which is probably in your course. Unlike any other battery I know it plays with three levels of oxidation of the metal. Electrons are withdrawn from Pb metal oxidising it to PbSO4 and furnished to reduce PbO2 at the other electrode - again forming PbSO4. This

    http://www.av8n.com/physics/lead-acid.htm#eq-dischargeminus

    Is a good article explaining it - and if you've been fed stuff like 'positive ions are attracted to the negative electrode' you may have the same puzzle that he seems a bit hung up on - HSO4- has to get to the negative electrode. Opposite to the electrical gradient he says. But then in the textbook Zn Cu SO42- cell, if you start with equal concentrations of SO42- on both sides, it moves against a concentration gradient becoming more concentrated on one side than another which never worries anyone. Shouldn't think about movement about an individual ionic species by itself. Surprised I don't find a better discussion, this thing must be well studied.

    The lead-acid battery is remarkable as the man says.



    Its disadvantages are so obvious we would drop it the moment we found anything better, yet it has lasted a whole century and a half.

    Finally according to recent researches the lead battery is explained by relativity theory.
    Electrons (in the semiclassical explanation given in link below, better than several I've seen) whizz around the lead nucleus so near light speed that they gain mass by relativity, and being heavier fall nearer the nucleus than they would, and so are less easily lost. Er, that's the opposite of what we want at the anode. But, they explain, it happens even more at the cathode, and this explains why the lead battery has a decent voltage and the tin equivalent doesn't. Why that is I can't explain. I might have said there are more electrons in PbO2 for it to happen to, but they do not say this. Electrons do not really rotate around the nucleus like in the pictures, even in Pb and you can't even imagine it for PbO2, but they do something a bit equivalent. Perhaps someone here can semi-explain this and other things a bit better

    http://www.economist.com/node/17899724
     
    Last edited: Jan 19, 2015
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