Chemistry - Gas Law Problems

Alright, I need some help understanding these gas law problems. I'll give you a few examples of the questions, and, if you could work them for me, and explain the steps so I can understand, that would be great. Thanks.

1. A gas has a volume of 2.0 Liters at a temperature of 20 degrees Celsius and a pressure of 1 ATM. If the temperature is increased to 100 degrees Celsius and the pressure remains the same, what will be the final volume of the gas?

2. 1.25 moles of hydrogen sulfide is heated to 500 Kelvin with a pressure of 15 ATM. Find the volume.

3. 14 cubic meters of gas at 490 Kelvin and 100 psi changes to 65 degrees celsius and 12 cubic feet. Find the final pressure.

Yes... that's about all for now. Any help would be GREATLY appreciated.

Ah, it's been a while since I've had to do any chemistry.

However, the gas laws I seem to recall as being fairly simple, so gimmie a moment to try and remember them/look them up.

Here we are.

Boyle's law: Pv=k (P=Pressure, V=Volume)
Charles's law: V/T =k
Gay-Lussac's law: P/T = k
Ideal Gas Law: Pv=nRT

So, looking at number 1:
It deals with a change in temperature, while pressure is constant. So we can ignore Boyle's and Gay-Lussac's laws.

So, what you should do is simply solve Charles's equation using the new temperature.

2:First, you need to know the molar volume of hydrogen. (If your notes don't tell you Wikipedia probably well.)
Then simply solve the ideal gas equation for volume. (In this case you have to re-arrange it to v=(nRT)/P)

3: First thing's first: Get all your measurements into the same units (Meters, Atmospheres, and Kelvins).
Then it should just be a case of solving the ideal gas equation. If not, you should be able to do it by solving the individual equations.

Hope that helps.

Originally Posted by GDwarf

2:First, you need to know the molar volume of

There is no such thing as the "molar volume" of a gas, and even if there was, you wouldn't need it for this problem. There's only one set of conditions, so all you have to do is plug into the ideal gas law and solve for the one remaining unknown.

As for question 3, the first thing you have to do once you have your units straight is plug the starting conditions into the ideal gas law and find the number of moles (which is almost always constant). Then you can plug in your ending conditions and solve.

Originally Posted by Snarwin

There is no such thing as the "molar volume" of a gas, and even if there was, you wouldn't need it for this problem.

You can express volume in terms of a molar volume; there are several ways to express variables, one of which is "molar." Usually you end up dividing the variable by the number of moles present. All it means is that you're saying how much volume you're taking up with a gas per mole.

Originally Posted by Snarwin

There is no such thing as the "molar volume" of a gas, and even if there was, you wouldn't need it for this problem. There's only one set of conditions, so all you have to do is plug into the ideal gas law and solve for the one remaining unknown.

As for question 3, the first thing you have to do once you have your units straight is plug the starting conditions into the ideal gas law and find the number of moles (which is almost always constant). Then you can plug in your ending conditions and solve.

I seem to recall a block in my chemistry class which was the "molar volume" of hydrogen.

However, you could be right that you don't need that for this problem, as I said, it's been some time.

Originally Posted by Snarwin

There is no such thing as the "molar volume" of a gas, and even if there was, you wouldn't need it for this problem.

Yes there is, it's just the volume occupied by one mole of gas under any given temperature and pressure. It's nothing that isn't already given by the ideal gas equation, though.

Originally Posted by GDwarf

First, you need to know the molar volume of hydrogen. (If your notes don't tell you Wikipedia probably well.)

Then simply solve the ideal gas equation for volume. (In this case you have to re-arrange it to v=(nRT)/P)

GDwarf, you accidentally told Andross how to solve it. :p The value is given by the equation v=RT/P.

Just the equation you gave but where n = 1.

Originally Posted by Jehanne

You can express volume in terms of a molar volume; there are several ways to express variables, one of which is "molar." Usually you end up dividing the variable by the number of moles present. All it means is that you're saying how much volume you're taking up with a gas per mole.

True, but the volume of a gas also depends on the pressure and temperature. Saying the molar volume, when talking about a gas, is meaningless.

Granted, I phrased my original post poorly, but my point remains the same.

I'd assume that just saying "a molar volume" means at 20C and 1 ATM.

Originally Posted by Snarwin

True, but the volume of a gas also depends on the pressure and temperature. Saying the molar volume, when talking about a gas, is meaningless.

I used to teach thermodynamics at the college level.



It's not meaningless -- there's just insufficient information provided above to make a clear statement. Additional information or assumptions are required. But that's the case with any ideal gas law problem.