Ideal Gas Law: messing with ratios

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Homework Help Overview

The discussion revolves around a problem involving the Ideal Gas Law, specifically focusing on the scenario of pumping air into a bicycle tire. The original poster attempts to determine how many additional moles of air are required to increase the gauge pressure from 1 atm to 5 atm, while assuming constant volume and temperature.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the application of the Ideal Gas Law and the implications of using gauge pressure versus absolute pressure in the calculations. There are attempts to clarify the correct interpretation of pressure units and their relevance to the gas law.

Discussion Status

Some participants have provided insights regarding the use of gauge pressure and its conversion to absolute pressure. The original poster acknowledges a misunderstanding and revisits their calculations, but still encounters issues with their results. The conversation indicates a productive exploration of the problem, with guidance being offered on the correct application of pressure definitions.

Contextual Notes

There is an ongoing discussion about the definitions of gauge pressure and absolute pressure, as well as the importance of using consistent units throughout the problem. The original poster's confusion about the required amount of moles versus the total moles is also noted.

Night-san
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Homework Statement



Air is pumped into a bicycle tire. The 43 moles of air initially in the tire have a gauge pressure of 1 atm. How many moles of air must be pumped into the tire in order to raise the gauge pressure to 5 atm? Assume that the volume and temperature of the air inside the tire are approximately constant.

Homework Equations



Ideal Gas Law:
PV = nRT


The Attempt at a Solution



Since the V and T were constant, as well as R, I set it up as (P1V1)/(n1T1) = (P2V2)/(n2T2) on the condition that R1 = R2.

V1=V2, T1=T2 So that would leave the equation at P1/n1 = P2/n2.

I decided to plug in and solve. P1=1 atm, n1=43 moles, P2=5 atm. 1/43 = 5/n2

When I solved for n2, I came out with 215 moles. My homework said it was wrong. can someone tell me where I went wrong?
 
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You are given the pressures in terms of gauge pressures. Are these the correct units to use with the ideal gas law?
 
SteamKing said:
You are given the pressures in terms of gauge pressures. Are these the correct units to use with the ideal gas law?


According to my instructor and the lessons/homework, the units should not matter as long as they are uniform throughout the problem and equations.
 
Definition: Gauge pressure refers to the pressure of a system above atmospheric pressure.

Gauge Pressure = Total Pressure - 1 atm.

[PLAIN]http://chemistry.about.com/o...ssure-Definition.htm[Í/url][/PLAIN] ehild
 
Last edited by a moderator:
Night-san said:
According to my instructor and the lessons/homework, the units should not matter as long as they are uniform throughout the problem and equations.

It's not a mater of units. Both absolute and gauge pressure are measured in the same units.
The gas law in the format you used is "designed" to work for absolute pressure. Same as it only works with temperature in Kelvin but not in Celsius.
 
Ok, I see the problem. thank you guys for the help. Much appreciated. ^_^
 
Well, I went back and re-did my work and came out with 129 moles. It was still wrong.
 
Nevermind, error on my part. I needed the amount of moles that was added, not the amount of moles in the end.
 

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