 Not logged in [Login ]
 Sciencemadness Discussion Board » Fundamentals » Beginnings » Easy P-chem HW question Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Detritus   » Test Forum Author: Subject: Easy P-chem HW question
National Hazard    Posts: 339
Registered: 1-7-2005
Location: Brothel
Member Is Offline

Mood: @%&$ing hardcore baby posted on 25-10-2008 at 21:11 Easy P-chem HW question Howdy.. I am having trouble with this question. A certain mixture of He and Ne in a 356-cm^3 bulb weighs .1480 grams and is at 20.0C and 748 Torr. Find the mass and mole fraction of He present. The equation that should be used is this PV = mRT/M where little m = mass and big M = Molar mass. From here m = PVM / RT m = (.9842 atm * .356 L * 4 g/mol) / (.0821 * 293K) m = .0583 grams The book says that the answer is suppose to be .0361 grams of He and .691 for mol fraction. Where am I messing up? I triple checked if I did the conversions right and I did. I am using the right gas constant too. (I like using L*atm). Thanks Magpie lab constructor     Posts: 5939 Registered: 1-11-2003 Location: USA Member Is Offline Mood: Chemistry: the subtle science. posted on 25-10-2008 at 21:34 m = your mass in grams, which is given. M = the molecular weight, which is your unknown. Solve for M, which will be a mole proportioned average for the two gases. Using M, determine the mole ratio of the two gases algebraicaly. [Edited on 25-10-2008 by Magpie] Magpie lab constructor     Posts: 5939 Registered: 1-11-2003 Location: USA Member Is Offline Mood: Chemistry: the subtle science. posted on 26-10-2008 at 14:35 Here's my solution in case you are having problems: M=mRT/(PV) = [(0.1480g)(0.0821 L-atm/(K-mole)(293K)]/[(0.9842 atm)(0.356 L)] = 10.16 g/mole For the mole fractions: Let X = mole fraction He then 4.003X + 20.18(1-X) = 10.16 -16.177X + 20.18 = 10.16; 16.177X = 10.02; X = 0.619 For the weights: mole ratio of He/Ne = 0.619/0.3806 = 1.626 Let W = weight of He then W/4.003 = 1.626[(0.1480-W)/20.18] W = 0.0477 - 0.3226W; 1.3226W = 0.0477; W = 0.0361g Did you perhaps transpose the last 2 digits of the mole fraction for the He? DeAdFX National Hazard    Posts: 339 Registered: 1-7-2005 Location: Brothel Member Is Offline Mood: @%&$ing hardcore baby posted on 27-10-2008 at 11:25

ahh thanks I kinda botched up on the second unknown equation.

I thought it was x + y = .1480g/10.16(g/l).

Right process wrong value :spaz:

 Sciencemadness Discussion Board » Fundamentals » Beginnings » Easy P-chem HW question Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Detritus   » Test Forum 