1. The measured volume of saturated Ca(IO3)2 solution was believed to be 1.0 ml
ID: 825934 • Letter: 1
Question
1. The measured volume of saturated Ca(IO3)2 solution was believed to be 1.0 ml and this volume was used in calculations, but due to improper use of a pipette, the actual volume delivered and used for titration was lower. Will the calculated solubility product constant of Ca(IO3)2 increase, decrease or remain unchanged. Justify.
I assume it does not stay the same, but I am unsure if it would increase or decrease and why.
2. Why does the saturated solution of calcium iodate have to be filtered before titration? HINT: Think about the equilibrium...
I know we filtered to remove the larger particles but I am not sure why else.
Explanation / Answer
The value of Ksp for an insoluble salt is determined experimentally using an
appropriate analytical method to measure the concentration of one or both ions in a
saturated solution of that salt. It is often easier to determine the concentration of one of
the ions, and then use the stoichiometry to find the concentration of the other ion. For
example, the dissociation reaction for calcium iodate (Equation 3) indicates that in a
saturated solution of Ca(IO3)2:
[Ca2+] = 0.5[IO3-] or [IO3-] = 2[Ca2+]
Thus, if you determine the concentration of iodate in the solution it is then easy to
calculate the calcium ion concentration and then Ksp. By substituting equation 5 into
equation 4, Ksp for Ca(IO3)2 can be expressed in terms of iodate alone:
Ksp = 0.5[IO3- ][IO3- ]2 = 0.5[IO3- ]3
volume is lowered, thus concentration will increase and thus, the Ksp will increase as per the above formulae.
2. The concentration of iodate ions is determined by redox titration. The titrant is a standard
solution of sodium thiosulfate (Na2S2O3). However, you are not titrating the iodate ions
directly. First, you must add potassium iodide and sulfuric acid to your sample of
saturated calcium iodate solution producing aqueous brown molecular iodine, I2(aq).
Next, the amount of iodine formed in the first step is determined by titration with sodium
thiosulfate in the presence of a starch indicator.
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