## anonymous one year ago Which statement best defines the combined gas law? A. At constant pressure, the volume of a gas increases or decreases by the same factor that its temperature increases or decreases. B. It defines a gas in which collisions between atoms or molecules are perfectly elastic and there are no intermolecular attractive forces. C. The law states that the product of the volume of a gas and its pressure over the temperature is equal to a constant. D. The total pressure exerted by a gas mixture is equal to the sum of the partial pressures of each individual gas in the mixture. Help! Is it C?

1. anonymous

But the equation for the combined gas law, I thought, was this: $P1V1\div T1 = P2V2\div T2$?

2. anonymous

P2V2÷T2 is not a constant...or is it? I am confused...

3. Photon336

I think this question is asking you like what assumptions about ideal gas behavior is this equation based off of. Let's look at the first case A. Here is our ideal gas law |dw:1441041131385:dw| With this equation we must make one of the variables constant if we want to study the other two. At constant pressure here's what out equation becomes: |dw:1441041207903:dw| You can see from here that at constant pressure that both volume and temperature are directly related. if you increase the volume by a certain factor the temperature goes up as well. same if you decrease it. A. is true but the question asks for the best explanation. like this equation there's more to it then just volume and temperature at constant pressure. you could study any of those relationships between volume, temperature, pressure, keeping one of those variables constant. this doesn't really tell us the whole story. B. I think this is true. because gases are assume to be point particles that they don't occupy any volume and they neither attract nor repel one another. the collisions are elastic means that, if I remember correctly there's conservation of mechanical energy I believe. like if you look at the ideal gas law it's this pV = nRT. That's basically all you need to know; and if you understand how to manipulate this formula you can derive boyles, charles, lussacs law and all that stuff. but the ideal gas law doesn't account for the attractions/repulsions between molecules. Also gas molecules take up space, Imagine if the gas particles themselves have volume, that means that the gas particles have less space to move around. if they have less space to move around, then they hit the walls of the container more and this affects pressure. that's why we subtract a constant from the total volume, which is like i guess the number of moles of gas*some constant. Also gases collide and repel/ the intermolecular attractions and these go up. as well and increase the pressure so that's why we introduce the term alpha. |dw:1441041733375:dw| I think B is the best answer because it's what the formula is built of of like the nuts and bolts behind it. C. $P _{1}V _{1}/T _{1} = nR$ That one is based off of the fact that P1V1/T1 = P2V2/T2. for this we keep the number of moles constant when we do this. so yeah this is true. D. Yeah this has to do with something called partial pressures. this also has to do with I believe ideal mixtures of gas. it assumes that the partial pressure exerted by each individual gas is proportional to it's mole fraction. mole fraction means the number of moles of a particular gas over the total number of moles. $P _{T}*n _{a} + P_{T}*n _{b} = P _{total}$ |dw:1441042271196:dw| mole fraction is just the number of moles of that gas over the total number of moles. $Gas A \frac{ 2 }{ 5 }*2 = \frac{ 4 }{ 5 } Atm$ = 0.8 $GasB = \frac{ 3 }{ 5 }*(2) = \frac{ 6}{ 5 } = 1.2$ so just to show you this is what the law of partial pressures tells us. 0.8+1.2 = 2.0 atm we add that up and that was the total pressure.

4. Photon336

@robinjane