Chapter 7 Solutions

Chapter 7 Solutions - Chapter 7 Answers 7) Heat is flowing...

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Chapter 7 Answers 7) Heat is flowing from the block of Mg to the water, until they equilibrate, that is. We will be using the relationship: q Mg =-q H2O mc Mg T=-mc H2O T For Mg: m=1.00Kg = 1000g c=1.024J g -1 ° C -1 T i =40.0 ° C T F =? For water: m=1.00Lx(1000mL/1L)x(1g/mL)=1000g c=4.18 J g -1 ° C -1 (this is a value you are expected to know from reading the chapter) T i =20.0 ° C T F =? It is important to note that, at equilibrium, Tf for water and Tf for Mg will be the same. Now we just substitute these values into our equation and solve for Tf to get: mc Mg T=-mc H2O T 1000g(1.024)(Tf-40)= - 1000g(4.18)(Tf-20) Tf=24.0 ° C *note* I am lazy with units because they tend to make the typed equations more difficult to follow I will provide the correct units for the variables list, and the correct units for the answer. I strongly suggest working through the units on your own as an exercise so that you can see where each unit comes from. 9) This is similar to the previous question, except we are solving for c glycerol . q Cu =-q gly mc Cu T=-mc Gly T For Cu: m=74.8g c=0.385J g -1 ° C -1 T i =143.2 ° C T F =31.1 ° C
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For glycerol: m=165mLx(1.26g/mL)=208g c=? J g -1 ° C -1 T i =24.8 ° C T F =31.1 ° C Mc Cu T=-mc Gly T 74.8g(0.385)(31.1-143.2)= - 208g(c)(31.1-24.8) C=2.46 J g -1 ° C -1 Molar heat capacity is found by multiplying by the molecular weight of glycerol (MW=92.1g/mol). Molar heat capacity=2.46 J g -1 ° C -1 x 92.1g/mol = 236 J mol -1 ° C -1 10) This one is hard to visualize at first. What you have to remember is that the water already in the calorimeter must remain at a constant temperature. Therefore, q=0 for this water (Tf-Ti=0). Now, we can focus on the two systems that actually change: the iron and the ‘added water’. q Fe =-q H2O mc Fe T=-mc H2O T For Fe: m=1.23Kg = 1230g c=0.449 J g -1 ° C -1 T i =68.5 ° C T F =25.6 ° C For water: m=? c=4.18 J g -1 ° C -1 (this is a value you are expected to know from reading the chapter) T i =18.5 ° C T F =25.6 ° C mc Fe T=-mc H2O T 1230g(0.449)(25.6-68.5)= -m (4.18)(25.6-18.5) m=800g However, it is important to note that the question asks for the volume of water, so we need to divide by the density (density=mass/volume). The density of water is 1g/mL, so 800g=800mL 17) According to the balanced reaction, each mole of methane releases 890.3 KJ of gas. a) With this knowledge, we can determine how many moles of methane yield 2.8x10 7 kJ: 2.8x10 7 kJ/890.3kJ mol -1 = 31450moles
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Molecular weight = mass/moles Mass=molesxMW =31450moelsx16g/mol =503.2Kg b) Ideal gas law! PV=nRT
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This note was uploaded on 04/04/2011 for the course SCIENCE CHEM 120 taught by Professor Fenster during the Winter '11 term at McGill.

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Chapter 7 Solutions - Chapter 7 Answers 7) Heat is flowing...

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