Question about the combined gas law?

The combined gas law is all three of those combined. It depends on what variables the question gives to you. If it gives you temp, volume, and pressure, you have to use the combined law. If it only gives you two, use the combined law, just cross out what is not given, and you'll end up with one of the three. The combined gas law was created to make it easy to remember one equation instead of three separate ones.

The combined gas law is all three of those combined. It depends on what variables the question gives to you. If it gives you temp, volume, and pressure, you have to use the combined law. If it only gives you two, use the combined law, just cross out what is not given, and you'll end up with one of the three. The combined gas law was created to make it easy to remember one equation instead of three separate ones.

let's do this.. let's start with the ideal gas law, and rearrange it a bit. PV = nRT PV/(nT) = R ok? But "R" is a universal constant.. 0.08206 Latm/moleK. isn't it? so ANY value of PV/(nT) must therefore equal ANY OTHER value of PV/(nT) and for two gases.. or 1 gas in two different states... P1V1 / (n1T1) = R = P2V2 / (n2T2) since they both = R, the must equal each other.. giving this equation... P1V1 / (n1T1) = P2V2 / (n2T2) ********** now.. let's consider the special cases where 1 or more of those variables is held constant.. P.. and.. n... for example.. so.. P1 = P2.. and.. n1 = n2... so let's substitute... P1V1 / (n1T1) = P2V2 / (n2T2) becomes P2V1 / (n2T1) = P2V2 / (n2T2) and that cancels to give V1/T1 = V2/T2 that... that special case where P and n is constant.. is called "Charles law" ********* here are more of those special cases with different things held constant (1)..[ P1V1 / (n1T1) = P2V2 / (n2T2) ] .. nothing held constant.. most general form.. no name! (2)..[ P1V1 / T1 = P2V2 / T2] .. n is constant.. this is called "combined gas law" (3)..[ V1 / T1 = V2 / T2 ] .. n & P constant.. "charles law" (4)..[ P1 / T1 = P2 / T2 ] .. n & V constant.. "no name".. erronously called "Gay Lussacs law" (5)..[P1V1 = P2V2 ] .. n & T constant... "Boyles law" (6)..[V1 / n1 = V2 / n2 ] .. P and T constant... Avogadros law etc... there are 14 different variations of this equation with different combinations of things held constant.. most are not named. ******* now.. where am I going with this you might ask? instead of memorizing combined gas law, charles law, boyles law, avogadros law, the conditions for which they apply and wasting valuable test time sorting through all of that STUFF to find the right equation for any given problem.. I always recommend you do this.. (1).. write down the equation P1V1 / (n1T1) = P2V2 / (n2T2) (2).. rearrange it for your desired unknown (3).. identify and cancel anything held constant (4).. plug and chug away.. READ my answers here where I work through actual problems http://answers.yahoo.com/question/index;_ylt=Asghij1aw4ZmWqcZeGMJ.1nty6IX;_ylv=3?qid=20120223144718AAC3EIX http://answers.yahoo.com/question/index;_ylt=AtJ9HL_LB62eP94bGGDMRfnty6IX;_ylv=3?qid=20120221114357AAGiCXx http://answers.yahoo.com/question/index;_ylt=AtJ9HL_LB62eP94bGGDMRfnty6IX;_ylv=3?qid=20120224041651AAI73DG notice how even though all of these problems are different... I still used that 1 equation? makes life simple doesn't it? while your classmates are trying to figure out which equation to use.. you'll be done and on the the next problem ***** this is basically what kendra was telling you except I left in the moles term.