combined gas laws?

Question

can someone explain to me in depth how to preform the combined gas laws i dont also know what p1 v1 = p2 v2
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t1 t2

Answer 1

What you presented here is not the combined gas law but the ideal gas law. Most gases under ordinary conditions follow the ideal gas law, sometimes written as:

P*V/T = n * Rm

P - Absolute pressure
V - Volume
T - Absolute teperature
n - Number of moles of gas
Rm - Universal Ideal gas constant (8.314 joule/(mole * K) or 0.00161 atm/(ft^3 * R))

If you are just seeing what happens to a fixed quantity of gas as it is compressed, heated, etc. You need not work with the number of moles or the gas constant since the right hand side of the equation will not change. In that case, you can compare a gas at one condition (with P1, V1, T1) to a gas at a second condition (P2, V2, T2) by the relation:

P1 * V1 / T1 = P2 * V2 / T2

In doing this, the pressure must be absolute pressure. Ordinary pressure gages measure the pressure above atmospheric pressure, called gage pressure. If you start with gage pressure, you need to add 14.7 psi (101 MPa) to the reading to get absolute pressure. The temperature must be absolute as well, for Fahrenheit, you need to add 459 to get the absolute temperature. For Celsius, add 273. With thiese corrections to and from absolute conditions, you can compare one state to another.

Answer 2

All that equation does is tell you how a gas will change if you do stuff to it. If you are expected to solve it, you will usually be told what five of those six. For example, what would be the pressure if we take a gas starting at a certain temperature, pressure, and volume and change it's volume and temperature to some other specific value. In that sense you just plug in the numbers and solve algebraically for the last variable.

Of course a good way to knock out two of those at a time is just to say that they're constant. For example, a problem might say that that you have a gas in a container of a fixed volume. That means you can just ignore all the V's in the equation. You'll never have TWO things that can't change, because that would mean the third thing can't change either!

Since the combined gas law is really a combination of a bunch of other laws, it usually simplifies down to one of them when you use it, or else you just get a pile of information to use it with. The important thing is just to use the same units for each side - if you start with volume in litres, you'll end with litres, and you may have to convert if they want some other unit. You must have the temperature in kelvin. And likewise if they are referring to the pressure, volume, or temperature of the sample indirectly, you'll have to find out what the actual pressure, volume, or temperature of the sample is - you can't use relative values.

Hope that helps!