Question

41 A solution is made 0.10 molar in Mo2+, 0.10 molar in NH3, and 1.0 molar in NH,CI. ill Mg(OH)2 precipitate? 42 How many grams of ammonium chloride must be added to 100 ml of 0.050-molar ammonium hydroxide to revent the precipitation of ferrous hydroxide, Fe(OH)2, when the NH4CI- NH4OH mixture is added to 100 ml of 0.020-molar FeCl2? Assume that the addition of solid NH4Cl produces no volume change 43 The Ksp of calcium phosphate, Ca3(PO4)2, is 1.3 X 10-32, and the third ionization constant of phosphoric acid is 2.2 × 10-13. Suppose that 0.31 g of calcium phosphate are added to 100 ml of water, and the pH of the solution is adjusted until all of the calcium phos- phate dissolves. What is this pH? (As- sume that HPO42- is the only other species formed in the solution and that CaHPO4 is soluble.)

Answer 1

Common ion effect: The common ion effect is observed when a weak acid is added to a salt which contain same an ion e.q. NH_3 (eq) + NH_4 Cl i.e. common ion effect is observed in buffer solution Due to common effect, PH of the solution decreases We have NH_3 + H_2 O rightleftarrow NH_4^+ + OH^- If we add NH_4 Cl to this solution the reduction of hydroxide ion concentration occurs, which lower the pH of the solution. Here, NH_4^+ is an ion that is common to the equilibrium (NH_4^+ is an acid) Given [Mg^+ 2] = 0.1m [NH_3] = 0.1M [NH_4 Cl] = 1.00M = [NH_4^+] If we mixed NH_3 and NH_4 Cl solution with one another the effect of dilution is cancelled out And we can use initial concentration of each spices for calculating pH of the solution since K_b = 1.8 times 10^-5 For NH_3 Now, K_b = [NH_4^+][OH^-]/[NH_3] = (1 + x)(x)/0.1 - x