3. Observations on addition of one drop of 6 M NaOH to Mg(NO3)2 solutionhice pre

Question

3. Observations on addition of one drop of 6 M NaOH to Mg(NO3)2 solutionhice predpi tate . Effect on solubility of Mg(OH) a. of added HCl solution b. of added NaOH solution of added NH, solution c. Explain your observations in Step 1. (Consider Reaction 8; how is it affected by addition of OH-ion?) In Step 2(a), how does an increase in [H] affect Reaction 3? (What does that do to Reaction 82) Explain your observations in Step 2(a). In Step 2(b), how does an increase in [OH-] affect Reaction 9? What does that do to Reaction 8? Explain your observations in Step 2(b). In Step 2(c), how does an increase in [NH,J affect Reaction 10? What does that do to Reaction 8? Explain your observations in Step 20c). In Step 3, you probably found that Mg(OH), was similar in some ways in its behavior to that of Zn(OH)2, but different in others. How was it similar? Explain that similarity. (In particular, why would any insoluble hydroxide tend to dissolve in acidic solution?) a. b. How was it different? Explain that difference. (In particular, does Mg2 appear to form complex ions with OH and NH,? What would we observe if it did? If it did not?)

Explanation / Answer

Solution:

When HCl is added, then the solubility of Zn(OH)2 (s) will increase. This is because when we add HCl, an acid, it will dissociate to give H+ and Cl- ions. These H+ ions will combine with the hydroxide ions OH- from the dissolution of zinc hydroxide, leading to the decreased concentration of OH- . According to Le Chatelier's principal, when the concentration of product is decreased, the equilibrium will shift to the product; more zinc hydroxide will be dissolved. The combined net ionic equation for the dissolution of Zn(OH)2 with the addition of HCl can be given by the following equations:

Zn(OH)2 (s) <===> Zn2+ (aq) + 2OH- (aq)

2H+ (aq) + 2OH- (aq) -----> 2H2O (l)

Zn(OH)2 (s) + 2H+ (aq) ===> Zn2+ (aq) + 2H2O (l)

Similarly, Mg(OH)2 solubility also increases on addition of HCl.

In case of Mg(OH)2 , aqueous ammonia precipitates white gelatinous Mg(OH)2 :

Mg2+ (aq) + 2NH3 (aq) + 2H2O (l) <===> Mg(OH)2 (s) + 2NH4+ (aq)

Ammonium salts dissolve Mg(OH)2 or prevent its precipitation, when added to aqueous ammonia.This is a buffer effect and shifts the pH to a lower value, causing a shift of the precipitation equilibrium to the left.

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