chemistry
posted by Kate .
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?

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 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...) 
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!
Respond to this Question
Similar Questions

thx 4 the help roger
thx 4 the help i got the steps You're welcome and I'm glad you understood the steps. Now use that strategy with your other word problems. Identify what the variable(s) are, convert them to equations, then solve the equations. Always … 
geometry
i don't get how to do slope i know the equation but it never comes out right...do you have any advice to help me b/c i ask some people and they don't get it either and we have a test tomorrow? 
Physics
Our teacher gave us a scenario we have to prepare to solve for a lab tomorrow: He has one of those toy flying airplanes suspended from the ceiling on a string of some length. He is going to charge it and start it spinning, and we have … 
Algergra
I am stumped! I have been working on these problems for the longest time and I can not figure out what I am doing wrong. Some how when I get to the last two steps I mess up, and I am beyond frustrated. The directions say: Write an … 
Math
I'm doing some word problems on finding the areas of rectangles, its just like all complicated. I don't understand how to do them. For example: A square field had 3 m added to its length and 2m added to its width. The area is 90 m2(squared). … 
Math 115
I need help with these problems 1. 18kg=______g I don't understand this. 2.1kg=______oz. can't figure this out. 3.20lb=______oz can't figure this out. 4.461mm=______m don't know where to begin. Can some please help me understand these … 
Chemistry (pleas help!)
a stressed chemistry student is hyperventilating into a paper bag. The student stops to wip his brow after a few breaths, and the bag is at 5.7 KPa and the temperature in the lecture hall is 24 degrees C. The bag is 0.75 L full. The … 
social studies
i am studying about palestinia and israel i have to write 8 bullets notes on what are the problems of the palestians and israel in the middle east. I don't understand the problems going on over there is there a website for my age 12 … 
Writing equations from a table
Hiiii! My teacher gave out some practice problems to help prepare for the big upcoming tEst.. I just don't remember hiw to do fhese problems.. Irs just hard for me.. these are not graded, I just need help so I can (hopefully) ace the … 
English
You are going to have a short quiz. I will hand out exam papers to you. Take one and pass the rest to the back. You have to solve the 10 Engish problems in 5 or 7 minutes. When you are finished with solving the problems, I will write …