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Recitation 2

Recitation 2 - Fall CHEM 110 Recitation 2 Group Problems 1...

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Unformatted text preview: Fall CHEM 110 Recitation 2 Group Problems 1. In the lab a substance is studied and the following observations are made. At room temperature (20C) the substance has the following properties: 1. It is a solid that is shiny and silver in color. 2. It has a density of 1.738 g/cc 3. It is malleable (it can be pounded into a flat sheet.) 4. It is ductile (it can be pulled into a thin wire). 5. It is a good electrical conductor. The substance is subjected to some further experiments and these additional observations are made. 6. It melts at 649C 7. It boils at 1105C. 8. It burns in air, glowing with an intense white light. 9. It reacts with bromine to give a brittle white solid Which of these characteristics are physical properties, and which are chemical properties? ------------------------------------------------------------------------------------------- 2A. How many protons, neutrons and electrons are present in 235U4+?What does the 4+ indicate? What does the number 235 indicate? 2B. Which of these species has 45 neutrons? A. 80Kr B. 80Br C. 78Se D. 79Kr E. 80Rb ------------------------------------------------------------------------------------------- 3. Natural uranium mined from the Earth contains the isotopes uranium-238, uranium-235 and uranium-234. The majority (99.2745%) of all the atoms in natural uranium are uranium-238. Most of the remaining atoms (0.72%) are uranium-235, and a slight trace (0.0055%) are uranium-234. Determine the average atomic weight of uranium. ------------------------------------------------------------------------------------------- 4. Which of the following processes is/are endothermic? i. A system absorbs 50 kJ of heat from the surroundings and does 30 kJ of work on the surroundings. ii. A system releases 55kJ of heat into the surroundings and 45 kJ of work is done on the system. iii. q = 12.5 kJ, w = 3500 J HINT: Calculate E = q + w Draw pictures using a box to represent the system and use arrows to show if energy is going into the system or out of the system. ----------------------------------------------------------------------------------------- Recitation 2 Fall CHEM 110 ----------------------------------------------------------------------------------------- 5. Rank these in order of increasing electrostatic potential energy. I an electron that is 2.8 nm from a +2 cation II an electron that is 2.8 nm from a -2 anion III an electron that is 2.8 nm from a proton A. I < III < II B. I = II < III C. II < III < I D. II < I < III E. III < II = I Hint: draw a picture to help you with this. -------------------------------------------------------------------------------------------------------- 6. What is the frequency of the wave shown below assuming it is light (electromagnetic radiation)? ------------------------------------------------------------------------------------------ 7. What is the frequency of light with an energy of 124 kJ/mol? ------------------------------------------------------------------------------------------- Challenge question: 8. An Argon laser gives off pulses of green light (wavelength = 514 nm). If a single pulse from the laser has a total energy of 10 mJ how many photons are in the pulse? Recitation 2 ...
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