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page 1 The Advanced Placement Examination in Chemistry 1970 to 2005 Solids, Liquids & Solutions
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page 2 1970 What is meant by the lattice energy of an ionic com- pound? What quantities need to be determined and how are they used to calculate the lattice energy of an ionic compound. Answer: Lattice energy - quantity of energy released in the formation of one mole of an ionic solid from its separated gaseous ions. The energy quantities needed to be determined: sublimation of solid metal ionization of gaseous atomic metal (ionization en- ergy) dissociation of gaseous non-metal ion formation by gaseous atomic non-metal (elec- tron affinity) They are used with Hess’s Law to determine the com- bination of gaseous ions. This is the Born-Haber Cycle. 1971 Solve the following problem AgBr (s) Ag + (aq) + Br - (aq) K sp = 3.3 × 10 -13 Ag + (aq) + 2 NH 3 (aq) Ag(NH 3 ) 2 + (aq) K = 1.7 × 10 +7 (a) How many grams of silver bromide, AgBr, can be dissolved in 50 milliliters of water? (b) How many grams of silver bromide can be dis- solved in 50 milliliters of 10 molar ammonia solu- tion? Answer: (a) [Ag + ][Br - ] = K sp = 3.3 × 10 -13 = X 2 X = 5.7 × 10 -7 M = [Ag + ] = mol/L AgBr that dissolve 0 . 050 L 5 . 7 1 0 - 7 mol A gBr 1 L 187 . 8 g A gBr 1 mol A gBr = = 5.4 × 10 -6 g AgBr (b) AgBr (s) Ag + (aq) + Br - (aq) K sp = 3.3 × 10 -13 Ag + (aq) + 2 NH 3 (aq) Ag(NH 3 ) 2 + (aq) K = 1.7 × 10 +7 ------------------------------------------ AgBr + 2 NH 3 Ag(NH 3 ) 2 + + Br - K = K sp × K = 5.6 × 10 -6 [Ag(NH 3 ) 2 + ] = [Br - ] = X M; [NH 3 ] = (10 - 2X) M K = [ Ag ( NH 3 ) + ][ Br - ] [ NH 3 ] 2 = X _ X ( 10 - 2 X ) 2 = 5 . 6 1 0 - 6 X = 2.4 × 10 -2 M = [Br - ] = mol/L dissolved AgBr (2.4 × 10 -2 mol/L)(187.8 g/mol)(0.050 L) = 0.22 g AgBr 1971 Molarity and molality are two ways of expressing con- centration. (a) Clearly distinguish between them (b) Indicate an experimental situation where express- ing concentrations as molarity is particularly ap- propriate. (c) Indicate an experimental situation where express- ing concentration as molality is particularly ap- propriate. Answer: (a) molarity (M) - molar concentration; composition or concentration of a solution expressed as num- ber of moles of solute per liter of solution. molality ( m ) - solution concentration expressed as number of moles of solute per kilogram of solvent. many possibilities, examples: (b) Acid - base titrations (c) Molecular weight determination by freezing point depression change. 1972 D (a) How many moles of Ba(IO 3 ) 2 is contained in 1.0 liter of a saturated solution of this salt at 25 ° . K sp of Ba(IO 3 ) 2 = 6.5 × 10 -10 (b) When 0.100 liter of 0.060 molar Ba(NO 3 ) 2 and 0.150 liter of 0.12 molar KIO 3 are mixed at 25 ° C, how many milligrams of barium ion remains in each milliliter of the solution? Assume that the volumes are additive and that all activity coeffi-
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