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Chemistry (including Biochemistry)/Chemical Equilibrium Gas Reaction In A Sealed Vessel


Hello, I hope this question falls within your area of expertise.

I'm currently studying for the MCAT and I ran into the problem below from the equilibrium chapter of the Kaplan book; I hope you'd be willing to take a look and explain one specific term for me.

in a sealed 1L container, 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to form .05 moles of NH3 at equilibrium. which of the following is the closest to the Kc of the reaction? 0.0001 is the answer.

Here's what I did: I balanced the equation then: Kc = [NH3]^2 / [N2] x [H2)^3 = [0.05]^2 / [1] [3]^3 = 0.0001

Then i realized that was wrong so I did this to find the number of moles consumed to from 0.05 mol of NH3:

0.05mol NH3 x (1 mol N2 / 2 mol NH3) = 0.025 mol N2
0.05mol NH3 x (3 mol H2 / 2 mol NH3) = 0.075 mol H2

But the book did this: 1-0.025 = 0.975 M (I have NO idea why they subtracted that number from 1, I've been trying to figure it out all day; Also I realize that 0.975M is close to 1M which is what we're given in the question but I have no idea how they arrived at 1-0.025).

3-3 x 0.025 = 2.925M (what is this term (3-3) and what's the reasoning behind it?)

Any help would tremendously appreciated; you have no idea how frustrated I got with this problem. Thanks so much in advance.


Hi Sara!

So you were on the right track: you determined that this is an excess problem, and then you figured out how much N2 and H2 you needed to make the 0.05 moles of NH3.

What the book did was this: since we have all of these gases in a sealed 1 L container before the reaction, the amounts of gases after the reaction are slightly different. So the book then calculated how much N2 was in excess, and how much H2 was in excess.

Since we started with 1 mol of N2 and reacted 0.025 mol of it, you then have 0.975 mol of N2 left. That's where the 1 came from.
Similarly, we can subtract three moles (starting) H2 minus 3 (mole ratio) times 0.025 moles of reacted H2 to get 2.925M. (That's where 3-3 came from.)

When we say Kc = [0.05mol NH3]/{[0.975 mol N2][0.2.925 mol H2]^3}, we get an answer of about 0.002

I suspect that this is a multiple choice question, so 0.001 would indeed be the closest. :)

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Trista Robichaud, PhD


No homework questions, especially ones copied and pasted from textbooks. I will answer questions about principles or give hints, but I do not do other's homework. I'm comfortable answering basic biochemistry, chemistry, and biology questions up to and including an undergraduate level of understanding. This includes molecular biology, protein purification, and genetics. My training/inclination is primarily in structural biology, or how the shapes of things affect their function. Other interests include protein design, protein engineering, enzyme kinetics, and metabolic diseases such as cancer, atherosclerosis, and diabetes. My chemistry weaknesses are that I do not know organic or inorganic synthesis well, nor am I familiar with advanced inorganic reactions. I will attempt quantum mechanics and thermodynamics questions, but primarily as they relate to biological systems. Furthermore, I cannot tell you if a skin photograph is cancerous, or otherwise diagnose any disease. I can tell you how we currently understand the basic science behind a disease state, but I cannot recommend treatment in any way. Please direct such questions to your medical professional.


I hold a PhD in Biomedical Science from the University of Massachusetts Medical School in Worcester. I specialize in Biochemistry, with a focus on protein chemistry. My thesis work involved the structure and functions of the human glucose transporter 1. (hGLUT1) Currently I am a postdoc working in peptide (mini-protein) design and enzymology at the University of Texas Health Science Center in San Antonio, Texas. I am in Bjorn Steffensen's lab (PhD, DDS), studying gelatinase A and oral carcinoma.

2001 American Association for the Advancement of Science
2007 American Chemical Society
2007 Protein Society
2011 UTHSCSA Women’s Faculty Association

Levine KB, Robichaud TK, Hamill S, Sultzman LA, Carruthers A. Properties of the human erythrocyte glucose transport protein are determined by cellular context. Biochemistry 44(15):5606-16, 2005. (PMID 15823019)
Robichaud TK, Appleyard AN, Herbert RB, Henderson PJ, Carruthers A “Determinants of ligand binding affinity and cooperativity at the GLUT1 endofacial site” Biochemistry 50(15):3137-48, 2011. (PMID 21384913)
Xu X, Mikhailova M, Chen Z, Pal S, Robichaud TK, Lafer EM, Baber S, Steffensen B. “Peptide from the C-terminal domain of tissue inhibitor of matrix metalloproteinases-2 (TIMP-2) inhibits membrane activation of matrix metalloproteinase-2 (MMP-2)” Matrix Biol. 2011 Sep;30(7-8):404-12. (PMID: 21839835)
Robichaud TK, Steffensen B, Fields GB. Exosite interactions impact matrix metalloproteinase collagen specificities. J Biol Chem. 2011 Oct 28;286(43):37535-42 (PMID: 21896477)

Poster Abstracts:
Robichaud TK, Carruthers. A "Mutagenesis of the Human type 1 glucose transporter exit site: A functional study." ACS 234th Meeting, Boston MA. Division of Biological Chemistry, 2007
Robichaud TK, Bhowmick M, Tokmina-Roszyk D, Fields GB “Synthesis and Analysis of MT1-MMP Peptide Inhibitors” Biological Chemistry Division of the Protein Society Meeting, San Diego CA 2010
Robichaud TK; Tokmina-Roszyk D; Steffensen B and Fields GB “Catalytic Domain Exosites Contribute to Determining Matrix Metalloproteinase Triple Helical Collagen Specificities” Dental Science Symposium. UTHSCSA 2011
Robichaud TK; Tokmina-Roszyk D; Steffensen B and Fields GB “Exosite Interactions Determine Matrix Metalloproteinase Specificities” Gordon Research Conference on Matrix Metalloproteinase Biology, Bristol RI 2011

Oakland University, Auburn Hills MI BS, Biochemistry 1998
University of Massachusetts Medical School, Worcester MA PhD, Biochemistry & Molecular Pharmacology 2001-2008
University of Texas Health Science Center, San Antonio TX Postdoc, Biochemistry 2009-Present

Awards and Honors
1998 Honors College Graduate, Oakland University
2009 Institutional National Research Service Award, Pathobiology of Occlusive Vascular Disease T32 HL07446
2011 1st Place, Best Postdoctoral Poster, Dental Science Symposium, UTHSCSA, April 2011

Past/Present Clients
Invited Seminars:
Robichaud TK, Fields GB. “Synthesis and Analysis of MTI-MMP Triple Helical Peptide Inhibitors” Pathology Research Conference, University of Texas Health Science Center San Antonio Pathology Department (June 18th, 2010)
Robichaud TK & Hill, B “How To Give A Great Scientific Talk” Invited Lecture, Pathobiology of Occlusive Vascular Disease Seminars, UTHSCSA (Nov 11th 2010), Cardiology Seminar Series, Texas Research Park (Feb 21st, 2011)
Robichaud TK; Tokmina-Roszyk D; Steffensen B and Fields GB “Exosite Interactions Determine Matrix Metalloproteinase Specificities” Gordon-Keenan Research Seminar “Everything You Wanted to Know About Matrix Metalloproteinases But Were Afraid to Ask” Bristol, RI (Aug 6th, 2011)

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