# Homework Help: Problem using the ideal gas law

1. Jan 2, 2010

### lm93

1. The problem statement, all variables and given/known data
In an internal combustion engine, air at atmospheric pressure and a temperature of about 20°C is compressed in the cylinder by a piston to 1/4 of its original volume (compression ratio = 4.0). Estimate the temperature of the compressed air, assuming the pressure reaches 20 atm.

2. Relevant equations
PV=nRT
restated as:(P1V1)/T1=(P2V2)/T2

3. The attempt at a solution
we are told that the initial situation: has a temperature of 20C or 293K, pressure of 1 atm. Set volume equal to 4m3
final situation: temperature=unknown, pressure=20 atm and volume=1m3

I tried putting these numbers in to the formula and got a few different incorrect answers depending on which units I used, such as: 100C, 52.5C, 58.6C

Thank you

2. Jan 2, 2010

### Staff: Mentor

Have you remembered to use absolute (Kelvin) temperature?

Show details of your calculation, results should not depend on the units used as long as you use same units for initial and final state, as they cancel out then.

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methods

3. Jan 2, 2010

### lm93

Oh I did forget to use absolute temperature! Thank you very much! I will try it again taking that into account...
So do I also have to use absolute pressure as well or is that not necessary?
If not then I would get P2=P1(T2/T1)
P2=101325N/m2(433K/293K)-->P2=149739.68N/m2

4. Jan 2, 2010

### Staff: Mentor

What would be initial pressure if you will use relative pressures instead of absolute ones?

5. Jan 2, 2010

### lm93

Oh...would it be zero?

6. Jan 2, 2010

### Staff: Mentor

Does it make sense?

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7. Jan 2, 2010

### lm93

I got 5662.6N but I'm not sure if it's correct.

8. Jan 2, 2010

### lm93

yes, because the relative pressure is the pressure minus the atmospheric pressure and since it is at atmospheric pressure then it would be zero.

9. Jan 2, 2010

### lm93

YESSSSSS I got it right! Thanks SO much!!