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EK Chemistry Lecture 30 minute question for Lecture 4!

Discussion in 'MCAT Study Question Q&A' started by masterMood, Jun 21, 2008.

  1. masterMood

    masterMood Banned Banned

    Jun 25, 2004
    Hi all,
    Number 79 page 145

    After filtering out excess solid, a student adds HCL to Solution 1 in Table 1. He then adds a small amount of CaSO4, which dissolves completely. Which of the following also occurs in the new solution?

    The answer is B) Undissociated HIO3 increases when the HCL is added.

    Some additional info that you guys might want to know is that Solution 1 is saturated.

    Why is it that when you add HCL (which dissociates to H+ and Cl-) it increases the amount of saturation possible? I thought Solution 1 was already saturated, so anything added to it would fall out of solution?

    Hmmm, I think I may have just answered my own question.

    Is it because 1) when you add more HCL you have to add it in liquid amounts (you can't just add HCl, you have to add it in a liquid form)

    2) With the reaction H+ (+) IO3- -><- HIO3, you cannot achieve saturation. Is this because there is no way for solid to occur? Because you're always adding more liquid if you're adding more acid (or base).
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  3. 161927

    161927 Guest

    Aug 5, 2007
    Of HIO3, HCl, and CaSO4, HIO3 is the least soluble species, because it is a weak acid. HCl is a strong acid and dissociates nearly completely. The effect of adding HCl is to increase the concentration of solute. The solution is satured with ions: H+, Cl-, Ca2+, SO4(2-) being the major species. HIO3 dissociates to a lesser extent upon addition of HCl because there is less available solvent to solvate/dissolve HIO3, due to the increased concentrations of ions in solution which is what really matters here. Does that sound right?
  4. 161927

    161927 Guest

    Aug 5, 2007
    On second thought, HIO3 isn't a weak acid. My explanation only works if HIO3 is less soluble / weaker acid relative to HCl, which still might be true.
  5. masterMood

    masterMood Banned Banned

    Jun 25, 2004
    But how would adding HCL increase the concentratino of solute, if the solution is already saturated?

    If the solution is saturated with H+ then wouldn't it be impossible to add more H+? And this depends on if the solution is even saturated with H+ or not. Is it saturated with H+ or just Ca2+ and SO4 2-?
  6. masterMood

    masterMood Banned Banned

    Jun 25, 2004
    Obviously, if we added more Ca2+ or SO4 2- it would just precipitate out out of solution (since those two common ions are definitely saturated).

    My confusion lies with whether or not the H+ + IO3- <---> HIO3 reaction is saturated too!

    The only way I am interpreting this is that by adding more acid, we CANNOT add more acid merely by adding it as a powder. The only way I've heard of adding more acid is by adding it in a liquid form (meaning the HCl is dissolved in water). So if we're going to add more HCl to the solution, we're also increasing the volume of water and giving room for the H+ ions to "fit in."

    Is my reasoning correct or flawed?
  7. engineeredout

    engineeredout 7+ Year Member

    May 11, 2008
    You were so right. That lecture 4 was terrible. I thought it was fine when going through it, and I did half the 1001 Q problems, but man when I took that 30 minute exam it was like I knew NOTHING
  8. StarStup

    StarStup 5+ Year Member

    Dec 13, 2007
    Penn and ID
    I was stumped on this question too...the only thing I got from their explanation was that they removed the excess solid so that the solution was not saturated anymore...? Not sure though...
  9. tco

    tco 7+ Year Member

    Apr 22, 2008
    I did every problem in the 1001 series on this lecture and understand every point made in the lecture, and still got an 8 on the exam. I don't even feel like reviewing it. I'm sick of this section.

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