# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: bany39 on May 09, 2016, 04:19:09 PM

Title: z=pv/nrt question
Post by: bany39 on May 09, 2016, 04:19:09 PM
Hi Dear Genius
there is question
with constant volume equal = 10 liter
what is constant of p.v in 200 bar ?
what is constant of p.v in 300 bar ?
what is constant of p.v in 400 bar ?
can anyone tell me how the z factor affect this constant(p.v) and show me the answer with 200 and 300 and 400 bar
thank you so much .
Title: Re: z=pv/nrt question
Post by: Babcock_Hall on May 09, 2016, 05:56:52 PM
Welcome to the forum.  It is a forum rule that you must show your attempt to answer a question before we can help you.
Title: Re: z=pv/nrt question
Post by: bany39 on May 09, 2016, 10:51:48 PM
Welcome to the forum.  It is a forum rule that you must show your attempt to answer a question before we can help you.
Ok ... it`s scuba diving question ...exactly with form of above .
Title: Re: z=pv/nrt question
Post by: billnotgatez on May 09, 2016, 11:02:36 PM
@bany39
I think you need to read the forum rules
Click on the link near the top center of the forum page.
Forum Rules: Read This Before Posting.
http://www.chemicalforums.com/index.php?topic=65859.0 (http://www.chemicalforums.com/index.php?topic=65859.0)

By the way
There are resources to learn about the ideal gas law
for example
https://en.wikipedia.org/wiki/Ideal_gas_law

As a side question
in the formula you presented there is the term t
which is probably temperature of the gas
Do they assume that temperature is the same in all instances?

by the way I found this while doing GOOGLE

http://chemistry.tcd.ie/staff/people/duesberg/ASIN/lecture%20notes/Chapter%203_%20Real%20Gases_annotated.pdf (http://chemistry.tcd.ie/staff/people/duesberg/ASIN/lecture%20notes/Chapter%203_%20Real%20Gases_annotated.pdf)
Title: Re: z=pv/nrt question
Post by: bany39 on May 09, 2016, 11:34:19 PM
thank you so much , yes the T is temperature and its constant for all instance .
Title: Re: z=pv/nrt question
Post by: bany39 on May 10, 2016, 12:55:02 AM
thank you so much , yes the T is temperature and its constant for all instance .
but maybe the question is wrong because every body just say read this article read that article and . but my question is simple and i say it again ....if the 10 liter tank with 200 working pressure 200 bar has the volume of 2000 liter air what is the volume of air for this tank in 300 and 400 bar ?? ( T and all others Items are constant ) is that too hard ???at the end of question say please give your answer with formula that you use it to solve it .
thanks again
Title: Re: z=pv/nrt question
Post by: Borek on May 10, 2016, 03:12:33 AM
z - in all cases - approximately equals 1. The way you wrote the equation is rarely used, as it can be used to show how far the gas is from the ideal gas approximation, which is a rather esoteric concept in the context of scuba diving. The most common form of the ideal gas equation is PV=nRT, but it is not really needed here.

All you need to calculate gas volume here is the Boyle's Law.
Title: Re: z=pv/nrt question
Post by: Enthalpy on May 10, 2016, 10:43:22 AM
In this case, the temperature remains near 300K, well above the constituents' critical temperature (N2 126K, O2 154K), and the attraction between air molecules stays small. The other imperfection of air is the volume of the molecules, which is neglected in the ideal gas theory, but is important at 300bar.

A simple way to check how important this volume is:
• As an ideal gas, a mole at 300K and 400bar would take 62cm3, hence 513kg/m3 for compressed oxygen for instance.
• But liquid oxygen weighs only 1141kg/m3 (at 1atm and 112K), and the liquid's volume resembles the molecules' volume
• So the volume of the gas under these conditions exceeds the one predicted by the ideal gas law.
Under these conditions, met as well in hydraulics pressure accumulators which use a bladder filled with nitrogen, PV hence exceeds nRT, so Z>1.

From Nist, at 300K:
200bar Z=1.06
300bar Z=1.15
400bar Z=1.26