KEY EXAM 2 SUMMER 2010 rev 2 - Exam 2 C112 Summer 2010 Answer key and solutions guide 1(a correct The highest vapor pressure is measured for pure water

KEY EXAM 2 SUMMER 2010 rev 2 - Exam 2 C112 Summer 2010...

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Exam 2 C112 Summer 2010 Answer key and solutions guide . 1. (a) correct. The highest vapor pressure is measured for pure water. Any dissolved ions or other solids lower the vapor pressure which is a colligative property. (b) through (e). wrong. See answer for (a). 2. Strategy: Use Π V = nRT to find moles, then use moles and mass of the sample to find molar mass. The volume of the water is found from mass and density: V = mass / D = 100.18 mL = 0.10018 L Use R given in the heading, with mmHg, and you don't have to change units. n = K ) 00 . 20 15 . 273 )( K mol / L mmHg 363 . 62 ) L 10018 . 0 )( mmHg 72 . 83 ( RT PV + = = 4.58768 x 10 - 4 moles These moles and the given mass refer to the same sample. Molar mass is a constant ratio regardless of sample size. mol 10 x 58768 . 4 g 5273 . 0 4 - = 1149.382g/mol 1149 g/mol ans (d) 3. Find total moles = 2+5 = 7 moles. Find mole ratio of water X H2O = total mol 7 O H mol 5 2 = 0.714285 Use equation in the heading: P gas = X gas P o gas = (0.714285)(450.9 mmHg) = 322 mmHg ans (b) 4. T freezing pt. = (K freezing pt. )(i)(conc.) = ( - 3.90 o C/m)(1.3)( 0.1450 moles/0.500 Kg) = 1.47 o C T freezing pt = T new - T original T new = 15.1 o C ans (d) 5. The solid lattice has to be "broken up" which required input energy of +789.0 kJ/mol (equal in magnitude and opposite in sign to energy released when the lattice forms, i.e., "lattice energy") When water forms spheres of hydration via electrostatic attractive forces around the ions, energy is released in the amount of - 792.0 kJ/mol. The sum of these two processes is the overall process of solvating an ionic solid. +789.0 kJ/mol + ( - 792.0 kJ/mol) = - 3 kJ/mol, ans (b) correct 6. Water flows from its higher concentration to its lower concentration, so water will flow from the 5 % solution (95% water) into the 20% solution (80% water) This will cause the piston in B to rise as solution B's volume increases, and conversely the piston in A will fall as A loses solvent volume. Answer (b) and (c) are correct.
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