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Unformatted text preview: Bodet (ngb299) Homework 2 sutcliffe (51630) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. NOTE: AFTER QUESTION 15 READ CAREFULLY! Some questions will require you to consider the vantHoff factor, i. If the solute is stated to be a nonelectrolyte, or if it is a sugar, i = 1. Assume complete dissocia- tion of ionic compounds. 001 10.0 points The molar heat of vaporization of carbon disulfide (CS 2 ) is 28.4 kJ/mol at its nor- mal boiling point of 46 C. How much energy (heat) is required to vaporize 5 . 7 g of CS 2 at 46 C? Correct answer: 2126 . 08 Joules. Explanation: 002 10.0 points How much heat energy is needed to raise the temperature of a 50 g sample of aluminum from 35 . 2 C to 90 . 5 C? The specific heat of aluminum is 0 . 897 J / g K. Correct answer: 2480 . 2 J. Explanation: m = 50 g SH Al = 0 . 897 J / g K T = 90 . 5 C- 35 . 2 C = 55 . 3 C = 55 . 3 K q = ? SH Al = q m T q = SH Al m T = (0 . 897 J / g K) (50 g) (55 . 3 K) = 2480 . 2 J 003 10.0 points Consider the following specific 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 of fusion for water is 334 J/g and the heat of vaporization for water is 2260 J/g. Calculate the amount of heat required to convert 51 g of ice at- 37 C completely to steam at 131 C. Correct answer: 160 . 765 kJ. Explanation: SH H 2 O(s) = 2 . 09 J / g C H vap = 2260 J/g SH H 2 O( ) = 4 . 18 J / g C H fus = 334 J/g SH H 2 O(g) = 2 . 03 J / g C m ice = 51 g T 1 =- 37 C T 2 = 131 C In this example, ice is converted to steam. Five separate steps take place here: 1) ice is warmed from- 37 C to 0 C; 2) a phase change from ice to water at 0 C; 3) water is warmed from 0 C to 100 C; 4) a phase change from water to gas at 100 C; 5) steam is warmed from 100 C to 131 C. Each of these steps involves a separate en- ergy calculation, and the sum of the energies of these five steps is the total amount of heat required for the process. For processes that involve warming a solid, liquid, or gas, we use the equation q = (SH) m T , where SH = specific heat, m = mass or moles, and T = temperature change. For phase changes (solid to liquid or liquid to gas), we use q = H vap or fus (m) , where again m = mass or moles. So for the first step, warming ice, we have q 1 = (51 g) 2 . 09 J g C C- (- 37 C) = 3940 J . For the second step, the phase change from ice to liquid: q 2 = (334 J / g)(51 g) = 17000 J . For the third step, warming water: q 3 = (51 g) 4 . 18 J g C (100 C- C) = 21300 J . Bodet (ngb299) Homework 2 sutcliffe (51630) 2 For the fourth step, the phase change from liquid to gas: q 4 = (2260 J / g)(51 g) = 115000 J ....
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This note was uploaded on 10/06/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.
- Spring '07