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Chemistry (including Biochemistry)/Ideal Gas Law Question - Pressure


A mixture of 39.5 g of ammonia and 27.2 g of xygen react in the following eaquation: 4NH3 + 3O2   -> 2N2 + 6H2O The intial and final temperature is 21.0 C and the volume in the container is 27.5l what is the pressure when the reaction reaches completion

This was on my first test i know the correct answer is 3.03 atm but i can 't figure it out I've used partial pressures limiting reagent and everything I know I just need to see it done.

Hi Jeo!

This week kicked my butt. How about you?

So this problem is actually simple but sneaky; it's nested problems. The first thing to do in any problem is to write out your variables, known and unknown.


V=27.5 L
T= 21.0 °C
R=ideal gas constant

... and you have a chemical reaction, which will produce some number of moles, n. Your best bet is the Ideal Gas Law, PV=nRT. If we're going to get pressure in atmospheres, we need an ideal gas constant in atmospheres. Look at your text to find an ideal gas constant with atmospheres and moles. I get this.

R=0.082057 [(Liter)*(atm)]/(moles*K)

To use the Ideal Gas Law, we need a) temperature in Kelvin and b) the total number of moles. Kelvin is easy, add 21 to 273 (0K) and you get 294K. Moles is a little more work, but we can make our lives easier by converting both ammonia and oxygen into moles right away by their molar mass.

n(NH3)=2.32 mol NH3
n(O2)=0.85 mol O2

Now we have a limiting reagent problem here. Fortunately the equation is balanced for us, less work. So we need to figure out which reagent is limiting using molar math. Usually you pick one reactant at random and try it to see how much of the other reactant you need to use the first one up. Let's try ammonia first.

2.32 mol NH3 * (3 mol O2)/(4 mol NH3) = 1.74 moles O2 needed to react that much NH3.

Shoot. We don't have that much O2. So it must be the O2 that is limiting. OK then.

Now we need to figure out how many moles TOTAL are in the flask, because ALL of them are going to push against the sides. (That's your partial pressures in action for total pressure.) We're going to react all the O2 (it's limiting), get some N2 and H2O, and have some NH3 left over. Let's do them all in turn, using the mole ratios from the equation above.

0.85 mol O2 * (2 mol N2)/(3 mol O2) = 0.57 mol N2
0.85 mol O2 * (6 mol H2O/(3 mol O2) = 1.7 mol H2O
0.85 mol O2 * (4 mol NH3/3 mol O2) = 1.13 mol NH3 (remember, we didn't use all the ammonia.)

ntotal = nO2 + nN2 + nNH3 + nH2O = 0 + 0.57 * 1.7 + 1.13 = 3.46 moles gas total. (But we're not done!)

We want PRESSURE. Now we can use the Ideal Gas Law. Let's solve for P.

P= nRT/V = {[0.082057 (liter*atm)/(mol*k)]*(3.46 moles)*(294 K)}/27.5L = 3.03 atm.

Let me know if you have any questions about how this was done. Basically, take the knowns you have, and figure out what you need to get to the end.


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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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