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Exam 1 key

# Exam 1 key - Chem 10 Exam 1 version A Useful information R...

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Chem 10 – Exam 1, version A Useful information: R = 0.08206 liter atm/ mol K = 8.3145 J/mol K 1 atm = 101325 Pa = 760 torr 1. At 100 ºC, a 1.20 L flask contains 42 mg of N 2 , 80 mg of O 2 , 5 x 10 -3 mol Ar and NO at a concentration of 2.50 x10 18 molecules/cm 3 . What is the total pressure in the flask? 42 mg N 2 = 1.5 mmol 80 mg O 2 = 2.5 mmol Ar is 5 mmol 2.5 x 10 18 /cm 3 = 2.5 x 10 21 /L, 2.5 x 10 21 /L x 1.2 L = 3 x 10 21 molecules = 5 mmol Total = n = 14 mmol = 1.4 x 10 -3 mol P= nRT/V = 0.357 atm 2. In the above question, what is the mole fraction of each O 2 ? 2.5 mmol O 2 / 14 mmol total = .178 3. A bicycle tire is filled to 130 psi at 22 ºC. It’s a cold day, and while riding, the temperature goes to -5 ºC. The tire also leaks and loses 7% of the air in it. What is the new pressure? Drop due to temperature a factor of (-5 + 273) / (22 + 273) = 0.9085 ( about a 9% drop) Drop due to leak is 7%, or the pressure remaining is 0.93 of the original. Final P = 130 psi x 0.9085 x .93 = 109.8 4. A 2.0 L stainless steel vessel is charged with 2.00 atm of hydrogen gas and 2.50 atm of nitrogen gas at 22 ºC. A spark ignites the mixture and it reacts, producing ammonia, NH

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