2 - cheng(erc656 Homework 2 sutclie(51630 This print-out...

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cheng (erc656) – 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 vant’Hoff 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 10.0points 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 . 4 g of CS 2 at 46 C? Correct answer: 2760 . 18 Joules. Explanation: 002 10.0points How much heat energy is needed to raise the temperature of a 42 g sample of aluminum from 25 . 4 C to 94 . 6 C? The specific heat of aluminum is 0 . 897 J / g · K. Correct answer: 2607 . 04 J. Explanation: m = 42 g SH Al = 0 . 897 J / g · K Δ T = 94 . 6 C - 25 . 4 C = 69 . 2 C = 69 . 2 K q = ? SH Al = q m Δ T q = SH Al m Δ T = (0 . 897 J / g · K) (42 g) (69 . 2 K) = 2607 . 04 J 003 10.0points 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 25 g of ice at - 17 C completely to steam at 137 C. Correct answer: 78 . 066 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 = 25 g T 1 = - 17 C T 2 = 137 C In this example, ice is converted to steam. Five separate steps take place here: 1) ice is warmed from - 17 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 Δ T , where SH = specific heat, m = mass or moles, and Δ T = temperature change. For 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 for the first step, warming ice, we have q 1 = (25 g) parenleftbigg 2 . 09 J g · C parenrightbigg bracketleftBig 0 C - ( - 17 C) bracketrightBig = 888 J . For the second step, the phase change from ice to liquid: q 2 = (334 J / g)(25 g) = 8350 J . For the third step, warming water: q 3 = (25 g) parenleftbigg 4 . 18 J g · C parenrightbigg (100 C - 0 C) = 10500 J .
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cheng (erc656) – Homework 2 – sutcliffe – (51630) 2 For the fourth step, the phase change from liquid to gas: q 4 = (2260 J / g)(25 g) = 56500 J . And, finally, for the fifth and last step, warming gas: q 5 = (25 g) parenleftbigg 2 . 03 J g · C parenrightbigg parenleftBig 137 C - 100 C parenrightBig = 1880 J .
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