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# Hw3ans_New - CHM 2045 THIRD ANSWER KEY INSTRUCTOR DON...

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Hw3ans_New.wpd 10/25/10 CHM 2045 THIRD ANSWER KEY INSTRUCTOR: DON SWIETER OFFICE: K-223 PHONE: 395-5303 QUESTIONS ABOUT GASES 1. a. b. c. d. e . f . 2. Since a torr is defined as a mm Hg at 0 °C and sea level, the mercury would be more dense (it would contract at the sub-zero temperatures at the north pole) and thus one would need to add mm Hg to the reading to get the correct pressure. Conversely, at the equator (here the temperature is much higher than zero and the mercury would expand) one would need to subtract mm Hg from the reading to get the correct pressure. 3. a. b. c. 4. a. b. c. 5 . a . b.

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2 c. 6 . 7 . 8. The easiest way to work this problem, perhaps, is to first figure out how many moles of He are left in the cylinder from the volume, temperature and pressure data. Then, after converting to mass of He, subtract from the original mass of He to find how much must be removed. The grams of He remaining, g = 36.6 g He & 7.54 g He = 29.1 g He released. 9 . 10. a. Since one mole of any gas at STP occupies 22.414 L we can calculate the density of ozone simply 33 by combining the molar volume of O (22.414 L/mol) with the molar mass of O (47.9982 g/mol). b. Therefore,
3 1 1 . Determination of empirical formula. ; Dividing by the smallest value gives, ; ; 22 Therefore, the empirical formula is CCl F . Determination of the molar mass involves using the gas density. Determination of the molecular formula. The molar mass of Z is 121 g/mol and therefore the molecular mass of Z is 121 amu/molecule. The formula unit, CCl F , has a formula mass of 121 amu/FU. The number of formula units per molecule can be calculated from these two numbers. Therefore, the molecular formula is CCl F also. 3 12. a. 3 H (g) + N (g) xxv 2 NH (g) b. At a fixed temperature and pressure (STP) the volume of a gas is proportional to the number of moles of gas. Since the mole ratio of N to H is 1:3, the gases will react in a volume ratio of 1:3 also. Thus 3.0 L of H will react with 1.0 L of N (3.0 L × a = 1.0 L). 32 c. Since the mole ratio of NH to H is 2:3, they will have a volume ratio of 2:3 also. Thus, 3.0 L 23 of H will form 2.0 L of NH . The conversion of this volume into moles is easy at STP. d. 2 ± N is Limiting Reagent.

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4 1 3 . a . and therefore, b. 2 c. Since the volume and the temperature of the system are fixed then the pressure of N is 22 proportional to the moles of N . Therefore, because the moles of N remained constant, the 2 pressure exerted by the N would remain constant at 500 torr. 2 But, the mole fraction of N would change. or
5 14. a. No, it will not form in the middle. It will form closer to the end with the slower moving gas. In 34 this case, HCl is heavier than NH and therefore is slower moving. Thus, the solid NH Cl will form closer to the end at which HCl was introduced. b. First calculate relative speeds. Note that the speed of a gas is proportional to the rate. Thus, Then calculate the relative distances traveled by each gas. Since the distance traveled by each gas is proportional to the speed of that gas then, The fraction of the total distance traveled by HCl is: Factoring out “distance of HCl” gives the fraction

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Hw3ans_New - CHM 2045 THIRD ANSWER KEY INSTRUCTOR DON...

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