HW 07-solutions

HW 07-solutions - cooper(mlc4285 HW 07 florin(55245 Correct...

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cooper (mlc4285) – HW 07 – florin – (55245) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points Why are two balls connected by a spring a good model for two atoms connected by a chemical bond? List all that apply separated by commas. If none are true, type “none”. A If the two atoms get farther apart than the equilibrium interatomic distance, they at- tract each other. B Each spring represents a real micro- scopic coiled metal wire that connects two adjacent atoms. If the two atoms get closer together than the equilibrium interatomic dis- tance, they repel each other. C The magnitude of the force one atom exerts on the other is proportional to the stretch or compression of the bond between them. Correct answer: A, C. Explanation: If the two atoms get farther apart than the equilibrium distance, the spring-like nature of the chemical bond kicks in and tries to prevent further separation. Thus, there is an attractive force set up between the two atoms. There are no actual springs or any other devices of any kind actually connecting the two atoms. The stretching or the compression sets up a force which acts against the stretch or the compression. This force is proportional to the amount of stretch or compression. 002 10.0points Consider two springs with spring constants k 1 = 220 N / m and k 2 = 590 N / m that are connect in series and have a 40 N weight attached to them. 220 N / m 590 N / m 40 N What total distance will the two springs stretch? Correct answer: 0 . 249615 m. Explanation: In a series system, the springs stretch a different amount, but each carries the full weight W = 40 N. W = k 1 x 1 x 1 = W k 1 = 40 N 220 N / m = 0 . 181818 m . In this same series spring system, the spring of constant 590 N / m stretch: W = k 2 x 2 x 2 = W k 2 = 40 N 590 N / m = 0 . 0677966 m . so the total stretch is Δ x total = 0 . 249615 m 003(part1of3)10.0points The diameter of a copper atom is approxi- mately 2 . 28 × 10 - 10 m. The mass of one mole ( N = 6 . 02 × 10 23 ) of copper is 64 grams. As- sume that the atoms are arranged in a simple cubic array. Remember to convert everything to S.I units. What is the mass of one copper atom in kg? Correct answer: 1 . 06312 × 10 - 25 kg. Explanation:

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cooper (mlc4285) – HW 07 – florin – (55245) 2 The mass of one mole of copper is M = 0 . 064 kg. The Avogadro number is N = 6 . 02 × 10 23 . The mass of each copper atom is m atom = M N = 0 . 064 kg 6 . 02 × 10 23 = 1 . 06312 × 10 - 25 kg 004(part2of3)10.0points How many copper atoms are there in a cubical block of copper that is 6 cm on each side? Correct answer: 1 . 82242 × 10 25 . Explanation: The length of each side of copper is L = 0 . 06 m. The diameter of each copper atom is d = 2 . 28 × 10 - 10 m. The no. of copper atoms is n Cu = parenleftbigg L d parenrightbigg 3 = parenleftbigg 0 . 06 m 2 . 28 × 10 - 10 m parenrightbigg 3 = 1 . 82242 × 10 25 005(part3of3)10.0points What is the mass of the cubical block of cop- per, in kg?
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