The Student Room Group

Deriving one ideal gas law from the other two

If you know pressure is inversely proportional to volume then P=c/VP = c/V
and volume is proportional to temperature so V=kTV = kT
then you can sub V=kTV = kT into P=c/VP = c/V and you get
P=c/kTP = c/kT which says that pressure is inversely proportional to temperature but this is wrong as pressure is actually proportional to temperature.
What am I doing wrong?
Reply 1
I believe your problem is arising from the fact you are trying to combine two equations which have different conditions on them:

P=c/V P = c/V is correct for constant temperature.

V=kT V = kT is correct for constant pressure.

You then try to combine the two which would only tell you a constant = constant, not a very significant answer.

You want to keep all P, V and T variable so the relation can only be simplified to: PV=cT PV = cT the problem here is you have 1 equation with 3 unknowns which you won't be able to solve.

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