Homework 2-solutions - tran (pt4954) Homework 2 sutclie...

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tran (pt4954) – Homework 2 – sutclife – (51045) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page – Fnd all choices be±ore answering. NOTE: A²TER QUESTION 15 READ CARE²ULLY! Some questions will require you to consider the vant’Hof ±actor, i. I± the solute is stated to be a nonelectrolyte, or i± it is a sugar, i = 1. Assume complete dissocia- tion o± ionic compounds. 001 10.0 points The molar heat o± vaporization o± carbon disulFde (CS 2 ) is 28.4 kJ/mol at its nor- mal boiling point o± 46 C. How much energy (heat) is required to vaporize 1 g o± CS 2 at 46 C? Correct answer: 372 . 997 Joules. Explanation: 002 10.0 points How much heat energy is needed to raise the temperature o± a 52 g sample o± aluminum ±rom 29 . 3 C to 91 . 6 C? The speciFc heat o± aluminum is 0 . 897 J / g · K. Correct answer: 2905 . 92 J. Explanation: m = 52 g SH Al = 0 . 897 J / g · K Δ T = 91 . 6 C - 29 . 3 C = 62 . 3 C = 62 . 3 K q = ? SH Al = q m Δ T q = SH Al m Δ T = (0 . 897 J / g · K) (52 g) (62 . 3 K) = 2905 . 92 J 003 10.0 points Consider the ±ollowing speciFc heats SH H 2 O(s) = 2 . 09 J / g · C , SH H 2 O( ) = 4 . 18 J / g · C , and SH H 2 O(g) = 2 . 03 J / g · C. The heat o± ±usion ±or water is 334 J/g and the heat o± vaporization ±or water is 2260 J/g. Calculate the amount o± heat required to convert 65 g o± ice at - 35 C completely to steam at 131 C. Correct answer: 204 . 625 kJ. Explanation: SH H 2 O(s) = 2 . 09 J / g · C Δ H vap = 2260 J/g SH H 2 O( ) = 4 . 18 J / g · C Δ H fus = 334 J/g SH H 2 O(g) = 2 . 03 J / g · C m ice = 65 g T 1 = - 35 C T 2 = 131 C In this example, ice is converted to steam. ²ive separate steps take place here: 1) ice is warmed ±rom - 35 C to 0 C; 2) a phase change ±rom ice to water at 0 C; 3) water is warmed ±rom 0 C to 100 C; 4) a phase change ±rom water to gas at 100 C; 5) steam is warmed ±rom 100 C to 131 C. Each o± these steps involves a separate en- ergy calculation, and the sum o± the energies o± these Fve steps is the total amount o± heat required ±or the process. ²or processes that involve warming a solid, liquid, or gas, we use the equation q = (SH) mΔ T , where SH = speciFc heat, m = mass or moles, and Δ T = temperature change. ²or phase changes (solid to liquid or liquid to gas), we use q = Δ H vap or fus (m) , where again m = mass or moles. So ±or the Frst step, warming ice, we have q 1 = (65 g) p 2 . 09 J g · C P b 0 C - ( - 35 C) B = 4760 J . ²or the second step, the phase change ±rom ice to liquid: q 2 = (334 J / g)(65 g) = 21700 J .
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This note was uploaded on 09/20/2011 for the course CALCULUS 7234832 taught by Professor Campsisi during the Spring '11 term at University of Texas at Austin.

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Homework 2-solutions - tran (pt4954) Homework 2 sutclie...

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