mdk2

I am a bit confused about a question in my textbook involving redox titrations. This is actually a sample problem that has been fully worked through in the book, so I am not trying to get its answer. However, I am confused about one particular step. Here is the question: A group of students prepares to standardize a Na2S2O3 solution. 32 mL of the Na2S2O3 solution is titrated into 50 mL of a 0.01 M KIO3 solution to reach the equivalence point. They first titrate the KIO3solution until it loses color, then add a starch indicator until the reaction is complete. The reaction proceeds in these two steps: IO3-+ I-+ H+ -----> I3-+ H2O I3-+ S2O32- -----> I-+ S4O62- Determine the concentration of the sodium thiosulfate solution at the beginning of the experiment.## I think I understand the basic logic here: We know that when we titrate a 32 mL solution of Na2S2O3 (unknown concentration) into 50 mL of 0.01 M KIO3 , we reach the equivalence point. To find the concentration of Na2S2O3 , we find the total number of moles IO3 at the equivalence point and then multiply it by the stoichiometric ratios to find the total moles of Na2S2O3 and then divide by the volume of Na2S2O3. I realize that the two reactions must be balanced. The textbook notes that it is essential that students not fall into the trap of balancing the first equation as follows: IO3-+ 2I-+ 6H+ -----> I3-+ 3H2O I realize that this wrong because the charges don't add up correctly. However, the textbook just follows this up by telling us the correct balance: IO3-+ 8I-+ 6H+ -----> 3I3-+ 3H2O. and I3-+ 2S2O32- -----> 3I-+ S4O62- What I don't understand is how we are supposed to figure out how to achieve this balance. I find myself trying to keep track of two different sets of variables (the stoichiometric balance and the charge balance) and can't conceive of a strategy for doing so that makes any sense. I have a two part question. First, generally speaking, is there a strategy/approach for achieving this balance? In stoichiometric atom balancing, we generally save Hs to last, for example. Is there a roadmap one can apply for balancing when we have to balance atoms and charges? And second, this question follows shortly after the textbook's discussion of balancing half reactions. However, I am struggling to see how this reaction links to half-reactions. Are the two equations above half reactions? Should I be balancing them as half reactions? (E.g. adding water to balance Os, adding H+s to balance Hs?) Or is that not at all the way I should be thinking about this? So, in sum, I am trying to figure out (a) how to keep track of balancing atoms and charges, (b) whether balancing the half-reactions is the answer, (c) if so, how, and (d) if not, what in the world half reactions have to do with any of this! Thank you so much! (PS: I know that the answer to this problem is 0.094 M -- I promise I'm not trying to get the answer! Just trying to understand the process!)

I am trying to understand the following question: The molar solubility of Fe(OH)3 in an aqueous solution was determined to be 4 x 10^-10 M. What is the value of Ksp for Fe(OH)3 at STP? I understand that Ksp = [Fe3+] [OH-]3. I know that we are meant to look at the mole ratios of products to reactants, substituting x to get Ksp = (x)(3x)3. And I see that we are meant to then reason that x = 4 x 10^-10 M. However, this is where I get lost: I thought that the whole point of the Ksp was to convey the relative amount of product vs reactant we have at equilibrium/saturation. If we have a very high Ksp, for example, I thought that it meant that the equilibrium favored the products, signifying high solubility (and low Ksp would favor the reactants, signifying low solubility.) If that is the case, then how can we assume that the concentration of Fe3+ is equal to the concentration of Fe(OH)3 at saturation? If the concentration of Fe3+ ions, for example, is always the same as the molar solubility of Fe(OH)3, then I don't see how the Ksp means anything at all. I would have thought that in a situation of very high Ksp, the [Fe3+] would be much higher than the molar solubility of Fe(OH)3 and that in a situation of a very low Ksp, the [Fe3+] would be much lower. But this question seems to want us to use simple stoichiometry. Am I missing something fundamental regarding the meaning of Ksp and its relationship with concentrations? Thank you so much!