Results of Experiment: (1) CaCl2(aq) + H2C2O4 *A white cloudy precipitate formed

Question

Results of Experiment:

(1) CaCl2(aq) + H2C2O4
*A white cloudy precipitate formed. Can't see through it.
(2) CaCl2(aq) + K2C2O4(aq)
*A thicker white precipitate than #1 formed.
(3) Results of (1) + 6M HCl
*Nothing occurred .
(4) Results of (3) + 6M NH3
*Precipitate thickened.
(5) CaCl2(aq) + 6M NH3
*No precipitate

..........................[1]
Ca2+ + C2O42- <----> CaC2O4(s)
+................[2]
H3O + NH3 <----> NH4+ + H2O
....^
[3] |
..../...................[4]
HC2O4- + H3O+ <----> H2C2O4 + H2O
+
H2O

(a) Account for the difference in behavior of H2C2O4 and K2C2O4 in tubes 1 and 2. Specifically, why does one solution form so much more CaC2O4(s)?

(b) From this array of coupled equilibria, explain the effect of adding HCL on these equilibria. (Note that H3O+ praticipates in equilibria 2,3, and 4.) Why does the addition of HCl change the amount of CaC2O4(s)?

(c) Consider the solutions 4 and 5, where NH3 was added. Does Ca(OH)2(s) form when NH3 is added to Ca(2+) solution (solution 5)? How does ammonia effect the amount of CaC2O4(s) present in solution 4?

(d) Why does the H3O+ concentration have more influence on the precipitation of the salt of a weak acid, such as CaC2O4(s), than on the precipitation of the salt of a strong acid, such as AgCl(s) or CaSO4(s)?

Explanation / Answer

a):(1) CaCl2(aq) + H2C2O4 ------>CaC2O4 + 2HCl,

*A white cloudy precipitate formed. Can't see through it. it was ppts of CaC2O4 but because of HCl it was not precipitated well.

(2) CaCl2(aq) + K2C2O4(aq) ----> CaC2O4(s) + 2KCl
*A thicker white precipitate than 1 formed as HCl was not formed in this reaction while salt KCl was formed

So, salt allowed the precipitation of CaC2O4 while HCl did not allowed and hence 2 reaction involves the formation of so much more precipitation.

b:3. CaC2O4 +HCl + 6M HCl    ------>CaC2O4 + HCl

Addition of HCl involves the more precipitation on its reaction with NH3 as it involves more salt formation as in reaction 4.

4. CaC2O4 +HCl + 6M HCl + NH3   ------> CaC2O4 + NH4+Cl-

c): No Ca(OH)2 was formed in the reaction 5 as shown

5: (5) CaCl2(aq) + 6M NH3   ------>  CaCl2(aq) + 6M NH3

d: This is because of the reason that C2O42- and Ca2+ are both strong base and acid respeveely while Ag+ is weak acid and Cl is strong base. So HCl does assit the precipitation of the first not secong

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