Question

Question 5 [11 marks] A seawater which serves as a useful commercial source of magnesium compounds contains a magnesium ion concentration of 0.054 mol dm? The magnesium is precipitated from solution by adding calcium hydroxide. Mg2+ (aq) + Ca(OH)2 (s) → Ca2+(aq) + Mg(OH)2 (s) a) Write an expression for the Ksp of Mg(OH)2, including its units. [3] b) [2] ) The numerical value for Ks is 2.00 x 10-11. Calculate [Mg2+ (aq)] in a saturated solution of Mg(OH)2. Hence calculate the maximum percentage of the original magnesium ions in the seawater that the precipitation method can rem remove. The magnesium ions in seawater are mainly associated with chloride ions. (1) Use the following AH values to calculate AH of the following reaction. MgCl2(s) -- Mg2+(aq) + 2Cl(aq) [2] Species ΔΗ, MgCl2(S) -641 Mg2+ (aq) -467 Cl(aq) -167 [2] (ii) Use your answer to explain why MgCl2 is very soluble in water. [2]

Answer 1

5 Given reaction is Mgrans tea (ora csi scath (aq) + Mg(OH)₂ (5) mol [mg+] = .00 osy drm3 I dm3 = I litre (a Now Iugory] =-054 mole mg (O)_ (s) dissociates Mg(OH)2 in the to following way ength +204 O S 25 tere Sa solubility of ug (on us in mol/ [mgte] s (25)² = 4s3 molt mois Ksp = [orig² aus3 mo13 (dm3) = 45² mol 3 dnia Now Ksp = 2x10' (from kesp = 45² 2x10" و دی isx10" = 501012 S- 4 (8x102)' . 1.7x10 mol/l fance [Mght cas ) = 5 = 1.7x10 mol/L

original concentration of ngth = .054 mol concentration of night ng(en) = 1.84164 mol/l 1.7x154 X100 % (ngth] removed from seawater = 054 = 0.32% o (d) Given rection ci mg cla(s) reaction Mgth (aq) +2cí (aq) Now enthal by change for the above DH = {OHI (Product) EDHI (Product) - Earf (reactant) = Ohi (Mgth)) + 2 OH (cicaa)) - Atif (mg clas) = -467 + 2 (-167) - (641) =-467 -334 +641 =-160 kJ/mol lange het ook mos) in mater. The fore nach (lii A heat is evolved (-160kg/mol) ngcha is dissolved in Soluble in water very Mli ori - 160k] KI/mole that heat is evolved. nidicales sign