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Equilibrium 1994A Teddy Ku

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1994 A MgF 2(s) Mg 2+ (aq) + 2F - (aq) In a saturated solution of MgF 2 at 18 degrees Celsius, the concentration of Mg 2+ is 1.21x10 3 molar. The equilibrium is represented by the equation above. a)Write the expression for the solubility-product constant, K sp, and calculate its value at 18 degrees Celsius. K sp = [Mg 2+ ] [F - ] 2 [F - ] = 2(1.21x10 -3 ) K sp = (1.21x10 -3 )(2.42x10 -3 ) 2 = 7.09x10 -9

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1994 A MgF 2 (s) Mg 2+ (aq) + 2F - (aq) In a saturated solution of MgF 2 at 18 degrees Celsius, the concentration of Mg 2+ is 1.21x10 3 molar. The equilibrium is represented by the equation above. b)Calculate the equilibrium concentration of Mg 2+ in liter of saturated MgF 2 solution at 18 degrees Celsius to which.100 mole of solid KF has been added. The KF dissolves completely. Assume the volume change is negligible. MgF 2 (s) Mg 2+ (aq) + 2F - (aq) Initial (M) Change (M) Equilibrium (M) x x-x x x 2.42x x 2.42x x K c = [Mg 2+ ] [F-] 2 = (1.21x x)(2.42x x) 2 = 7.09x10 -9 Solve for x on calculator X= x10 -3 [Mg 2+ ] = 1.21x x = 2.42x10 -3

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1994 A MgF 2 (s) Mg F - (aq) In a saturated solution of MgF 2 at 18 degrees Celsius, the concentration of Mg 2+ is 1.21x10 3 molar. The equilibrium is represented by the equation above. c)Predict whether a precipitate of MgF 2 will form when milliliters of a 3.00x10 -3 molar Mg(NO 3 ) 2 solution is mixed with milliliters of a 2.00x10 -3 molar NaF solution at 18 degrees Celsius. Calculations to support your prediction must be shown. Set up the equilibrium law and calculate (did in a) –Ksp=7.09x10 9 Calculate ion product –(3.00x10 -3 )(2.00x10 -3 ) 2 =1.2x10 -8 Compare ion product with K sp –If the ion product > Ksp, the solution is supersaturated, and precipitate will form. –If the ion product is < or = to the Ksp, no precipitate will form. –1.2x10 -8 < 7.09x10 9 No precipitate

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1994 A MgF 2 (s) Mg F - (aq) In a saturated solution of MgF 2 at 18 degrees Celsius, the concentration of Mg2+ is 1.21x10 3 molar. The equilibrium is represented by the equation above. d)At 27 degrees the concentration of Mg 2+ in a saturated solution of MgF 2 is 1.17x10 -3 molar. Is the dissolving of MgF 2 in water an endothermic or an exothermic process? Give an explanation to support your conclusion Equilibrium shifts to the left when temperature increases –Heat must be a product Exothermic if heat is a product

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