I know how to use the equations pv=nrt and P1xV1/n1xT1 = P2xV2/n2xT2, but I don't understand when to use them.

I have a test tomorrow and I'm going over some word problems, but I can't figure out how decide which formula you need to use. Is there a specific way to know?

I was thinking that if, for example, two different pressures were given, two different volumes were given, two different temperatures were given and only one mole, then you use P1V1/n1T1...etc to find the missing mole.

But, if only one pressure, one volume, and one temperature were given and you needed to find moles, then you use pv=nrt.

That's how I tried figuring out the difference, but I was afraid there was some exception to throw me off (which usually always happens on tests...)

Honestly, I've never used P1V1/n1T1 = P2V2/n2V2.

I think PV = nRT will solve all of them. If all except one variable are given out of P1, P2, V1, V2, T1 and T2, then I use P1V1/T1 = P2V2/T2 (because it's a little faster) but I could use PV = nRT twice; once to solve P1V1/R*T1 for n, then use PV = nRT again with that n and P2, V2, for example, and solve for T2. So I don't know that it matters which you use. Check my thinking.

I think your thinking is OK. I tried making up a P1, V1, T1 and calculating n1. Then I made up a P2, V2, T2 and calculated n2, both calculations from PV = nRT.

Then I used the old P1, V1, T1, and the calculated n1 and the old P2, V2, and calculated n2 and calculated T2 from the P1V1/n1T1 = P2V2/n2T2 equation and got the same answer. In fact I tried it with two or three sets of numbers and I always came out with the same answer no matter which way I solved it. Of course, if only ONE P, V, and T are given, you have no choice but to use PV = nRT.

Okay, good. I think I'm all set then. Thank you!

When solving word problems involving gases, it is important to determine which formula to use. Here is a step-by-step guide to help you decide when to use the equations pv = nrt and P1V1/n1T1 = P2V2/n2T2:

Step 1: Determine what information is given in the problem. Look for known quantities such as pressure, volume, temperature, and number of moles.

Step 2: Identify what the problem is asking you to find. Are you solving for pressure (P), volume (V), temperature (T), or number of moles (n)?

Step 3: Check if the given information includes any of the variables in the ideal gas law equation pv = nrt or the combined gas law equation P1V1/n1T1 = P2V2/n2T2. If all the variables are given, you can use either equation, depending on which one is more convenient or comfortable for you to work with.

Step 4: If not all the variables are given, use the information given in the problem to manipulate the equation in order to solve for the desired variable. This may involve rearranging the equation or using additional information to find missing variables.

For example, if you are given pressure, volume, and temperature, and you need to solve for the number of moles (n), you would use the ideal gas law equation pv = nrt. Rearrange the equation to solve for n: n = (Pv)/(rt). Plug in the given values for P, v, and T to calculate n.

If you are given the initial pressure, volume, and temperature, and the final pressure and volume, and you need to solve for the final temperature, you would use the combined gas law equation P1V1/n1T1 = P2V2/n2T2. Rearrange the equation to isolate T2: T2 = (P2V2T1)/(P1V1). Substitute the given values for P1, V1, P2, and V2 to calculate T2.

Remember to always double-check your units and convert them appropriately if necessary. Pay attention to any specified units or conversions mentioned in the problem.

Practicing different types of word problems and actively applying these steps will help you develop intuition for which formula to use in different scenarios.