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Mid-term Key CHEM 131 - Spring 2009

Mid-term Key CHEM 131 - Spring 2009 - Chemistry 131 Spring...

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Chemistry 131 – Spring 2009 CHEM 131 Midterm Exam Wednesday, April 22, 2009 5:00 – 7:00 PM Men’s Campus 4:00 – 6:00 PM Women’s Campus Student Name: _________________________________ Student ID N°: ______________ Instructions : 1. Please read the following text. By writing your name and student ID at the top of this page you are agreeing to accept these rules for taking this exam. “I pledge that I have neither received nor offered any unauthorized assistance during the completion of this exam.” Any incident will be reported to the Student Affairs Office . 2. Spend a few minutes to read through the whole paper carefully. 3. Questions have varying points, as shown in the table below. Plan the use of your time carefully so that you will be able to work on all the questions in the allotted time. 4. This exam paper has a total of 8 pages, numbered 1 to 8. You are also provided with a data sheet. Please do not write on the data sheet. For full credit on the problems, you must show all work , and if applicable, give the answer to the correct number of significant figures . Points Possible Score Points Possibl e Score Points Possibl e Score Question 1 8 Question 8 6 Question 15 2 Question 2 10 Question 9 8 Question 16 4 Question 3 4 Question 6 Question 17 4 1
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Chemistry 131 – Spring 2009 10 Question 4 6 Question 11 6 Question 5 12 Question 12 8 Question 6 4 Question 13 4 Total 100 Question 7 4 Question 14 4 1. Nitrogen forms several gaseous oxides of general formula NO x . One of them has a density of 1.33 g/L measured at 764 mmHg and 150 ° C. Write the molecular formula of the compound. Molar Mass = d R T / P = (1.33 g/L × 0.082058 L atm mol –1 K –1 × 423 K)/(764/760 atm) = 45.9 g/mol 45.9 g/mol = 14.007 gmol + ? × 15.994 g/mol ? = 2 NO 2 + 3 for moles of gas mistakes minus 1 or 2 pts math error + 3 for MM of gas incorrect units/conversions + 2 for x = 2 2
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Chemistry 131 – Spring 2009 2. A mixture of He and Ne gases is collected over water at 28.0 ° C and 745 mmHg. At 28.0 ° C, the vapor pressure of water is 28.3 mmHg. If the partial pressure of He gas is 236 mmHg, calculate the partial pressure of Ne. 745 mmHg = P He + P Ne + P water = 236 mmHg + P Ne + 28.3 mmHg P Ne = 481 mmHg Minus 1 pt missing or wrong units Minus 0.5 pt incorrect significant figures Minus 2 for incorrect calculation 3. When a gas is compressed from 25.0 L to 20.5 L in volume, it releases 3650 J of heat to the surroundings. If the internal energy of the gas decreases by 3250 J at the same time, calculate the pressure being placed on the gas.
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