Ideal Gas Law: messing with ratios

AI Thread Summary
To determine how many moles of air are needed to raise the gauge pressure in a bicycle tire from 1 atm to 5 atm, the Ideal Gas Law is applied, considering constant volume and temperature. The initial calculation incorrectly focused on the total moles instead of the additional moles required. The distinction between gauge pressure and absolute pressure is clarified, emphasizing that the Ideal Gas Law requires absolute pressure values. After correcting the approach, the correct answer involves calculating the difference in moles rather than the final total. The discussion highlights the importance of understanding pressure units and the specific requirements of the gas law.
Night-san
Messages
5
Reaction score
0

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?
 
Physics news on Phys.org
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.
 

Similar threads

Calculating ΔE Difference for 2 Samples of Monatomic Ideal Gas
Replies
4
Views
2K
Ideal Gas Law: Question about a compressor exam question
Replies
4
Views
2K
Ideal Gas Law and Pressure at 80°C
Replies
2
Views
3K
Thermodynamics problem (ideal gas law, kinetic theory, processes, etc.)
Replies
18
Views
1K
Calculations using the Ideal Gas Law
Replies
2
Views
2K
What would be a real-life example of the ideal gas law?
Replies
3
Views
9K
Open Gas Turbine - calculate T2, T3, T4, efficiency
Replies
1
Views
2K
Verification of ideal gas law experiment
Replies
5
Views
1K
Calculating Hydrogen Mass from Ideal Gas Law
Replies
3
Views
4K
Proof question related to the Ideal Gas Law
Replies
8
Views
2K

Hot Threads

Recent Insights

Back
Top