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Unformatted text preview: hernandez (ah29758) Homework 2 sutcliffe (51045) 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 7 . 8 g of CS 2 at 46 C? Correct answer: 2909 . 38 Joules. Explanation: 002 10.0 points How much heat energy is needed to raise the temperature of a 50 g sample of aluminum from 20 . 6 C to 93 C? The specific heat of aluminum is 0 . 897 J / g K. Correct answer: 3247 . 14 J. Explanation: m = 50 g SH Al = 0 . 897 J / g K T = 93 C- 20 . 6 C = 72 . 4 C = 72 . 4 K q = ? SH Al = q m T q = SH Al m T = (0 . 897 J / g K) (50 g) (72 . 4 K) = 3247 . 14 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 46 g of ice at- 14 C completely to steam at 137 C. Correct answer: 143 . 353 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 = 46 g T 1 =- 14 C T 2 = 137 C In this example, ice is converted to steam. Five separate steps take place here: 1) ice is warmed from- 14 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 137 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 = (46 g) parenleftbigg 2 . 09 J g C parenrightbigg bracketleftBig C- (- 14 C) bracketrightBig = 1350 J . For the second step, the phase change from ice to liquid: q 2 = (334 J / g)(46 g) = 15400 J ....
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- Spring '07