chapter 10 gases Flashcards
(16 cards)
Which of the following would best explain why the pressure of gas would increase as the volume is reduced at constant temperature?
a. increase in the average speed of the molecules
b. an increase in the frequency of the molecules colliding with the walls of the container
c. an increase in the average force of collisions with molecules with the walls of the container
d. an increase in the frequency and average force of collisions of molecules with the walls of the container
e. decrease in the average speed of the molecules
b.
I guessed and got this right. This is one of those conceptual gas question that I really hate but…
a. would be like if the temperature was increased
c. I think this again would be more like if the temperature were increased
d. this was my 2nd pick. But again, it includes the force increasing which I think would mostly happen if the temp were increased
e. this was the obvious rule-out
The book actually explains it well here:
As the temperature remains constant the average speed of the molecules will not change and therefore the average force of the collisions of molecules with the walls of the container will not change either.
What is the pressure of the gas trapped inside the mercury manometer in the following diagram?
This is hard to imagine without the picture but just try to draw it. It’s the U-shaped thing fluid between the two sides. The right upper side is closed and there’s a bubble where the gas in question is supposed to be. The left upper side is open and exposed to 1 atm of pressure. And on the left side, there’s 19 cm of difference pushing down.
hint: you have to know that 760 mmHg = 1 atm
a. 950 mmHg
b. 190 mmHg
c. 570 mmHg
d. 779 mmHg
e. 741mmHg
a.
So basically, if there were no difference in the level of fluid, the gas would have a pressure of 1 atm
But since there’s a difference of 19 cm (or 190mm) on the gas side, the gas is pressure is going to be more than 1 atm. How much more? Well… 1 atm (or 760 mmHg) + 190 mmHg = 950 mmHg
Which of the following is not an assumption of kinetic molecular theory?
a. in a sample of gas, the volume of the molecules is insignificant
b. the volume of gas is proportional to the number of moles of gas
c. the average kinetic energy of the gas molecule is proportional to temperature
d. all collisions are elastic
e. there are no attractive forced between molecules
It’s b.
I guessed and I guessed wrong. I knew it was between a. and b.
This is a messed up question as the book clearly states:
This is tricky as these are all true statements but only 4 out of the 5 are assumptions of kinetic molecular theory.
It turns out that the volume of a gas is proportional to the number of moles of gas at constant pressure and temperature (this is Avogadro’s Principle) but this is not an assumption made in kinetic molecular theory. The assumptions of Kinetic Molecular Theory are listed below.
Ideal Gas Theory (Kinetic Molecular Theory) Assumptions:
1) Gas molecules don’t have volume.
2) Gas molecules don’t have any attractive forces between themselves.
3) All collisions are elastic (no loss of kinetic energy).
4) A gas is composed of a large number of atoms/molecules in random motion.
5) KEavg T (KEavg = 3/2RT)
Which of the following is FALSE for gases behaving ideally?
a. Pressure is inversely proportional to temperature at constant volume.
b. Volume is proportional to temperature at constant pressure.
c. Volume is inversely proportional to pressure at constant temperature.
d. Volume is proportional to the number of moles of gas at constant pressure and temperature.
e. Pressure is proportional to the number of moles of gas at constant volume and temperature.
a.
Ugh. This takes a lot of thinking but you can figure it out. Rather than write of PV = nRT and adjust the equation, I found it easier to write something like P (up arrow) and V (down arrow) and then ask, is it true?
What is the volume of 3 moles of He behaving ideally at 298K and 2.5 atm?
(Use R = 0.0821L∙atm/mol∙K)
a. (3)(0.0821)(298)
2.5
b. (2.5)(0.0821)(298)
3
c. (2.5)(0.0821)
(3)(298)
d. 3
(2.5)(0.0821)(298)
e. 298
(2.5)(0.0821)(3)
a.
You just take PV = nRT and switch the equation around to solve for volume so…
V = nRT/P
What is the volume of 14 grams of N2 behaving ideally at 100 degrees C and 800 torr?
(Use R = 0.0821L∙atm/mol∙K)
a. (14)(0.0821)(100)
800
b. (0.5)(0.0821)(373)
800
c. (0.5)(0.0821)(373)(760)
800
d. (0.5)(0.0821)(373)(800)
760
e. (0.5)(0.0821)(100)(760)
800
c.
This is a really easy one to screw up on because no one would ever write the equation this way. I would convert 800 torr to atm before doing and stick it in the denominator (not have the conversion built into the fraction.
But basically, look at it like this…
(x)(x)(x) (x)(x)(x)(760)
———– ————–
1 (760)
————- = ———————
800 800
————– —————–
760 760
Cancel the 760’s in the denominators and you’re left with answer c.
For a sample of gas behaving ideally, the volume was quadrupled and the temperature doubled. What was the effect on the pressure?
a. It was 1/8 the original pressure
b. It was 1/2 the original pressure
c. It was double the original pressure
d. It was 4 times the original pressure
e. It was 8 times the original pressure
b.
This question is hard but if you sit there and think about it, you can figure it out.
First, I needed to figure out which equation to use. Spoiler: it’s just the ideal gas law re-written to solve for pressure (this would put volume in the denominator)
So if P = temp(2)/volume(4)
when you reduce, you get 1/2
Which of the following would have the highest density under identical conditions (temperature, pressure, and volume)?
a. He
b. Ne
c. Ar
d. Kr
e. the densities are the same for all of these
d.
Well, I guessed and I guessed wrong. I was thinking of the first part of kinetics theory that says gas molecules have negligible volume but I guess it’s talking about the volume of the molecules themselves as opposed to the molecules of a container (wish for all the hours I’ve watched and studies this, they could have made that a little clearer by now). But anyway, I guess ‘always remember that mass, volume, and density are all different but all related to each other by this equation:
Density = m/V (m = mass)
If these gases are all under the same conditions (temperature, pressure, and volume) then the same number of moles of gas would be present for each gas as well according to the Ideal Gas Law: n = PV/RT.
Each gas has the same volume and the same number of moles. With the same number of moles the gas with the highest molar mass will have the greatest mass and therefore the greatest density. Therefore Kr has the highest density.
A 32 grams sample of gas occupies 11.2L at a pressure of 2 atm and a temperature of 0 degrees C
What is its molar mass?
a. 4 grams
b. 8 grams
c. 16 grams
d. 32 grams
e. 64 grams
d.
So this one isn’t as bad as it looks. There’s a formula under ‘additional gas laws’ specifically called the ‘gas density law’ where you can find the molar mass (indicated by a capital M)
It’s
d (density) = m (mass)/V = PM/RT
So you can just rearrange the formula to solve for M
It looks like a 32 gram molar mass would be sulfur
A 7 grams sample of gas occupies 22.4 L at a pressure of 0.5 atm and a temperature of 273 C
What is the molar mass?
a. 7 grams
b.14 grams
c. 21 grams
d. 28 grams
e. 42 grams
d. 28 grams
They try to trick you with the temperature but remember to convert it to kelvin
So this one isn’t as bad as it looks. There’s a formula under ‘additional gas laws’ specifically called the ‘gas density law’ where you can find the molar mass (indicated by a capital M)
It’s
d (density) = m (mass)/V = PM/RT
So you can just rearrange the formula to solve for M
An 80g sample of a gas occupies 11.2L at a pressure of 10atm and a temperature of 0⁰C. Which of the following could be the identity of the gas?
a. H2
b. He
c. CH4
d. Ar
e. SO2
c.
This is kind of a fun question. I’m just going to copy and paste the explanation right from the quiz:
To identify the gas we will need to calculate its molar mass. The molar mass of a substance is calculated as m/n. We’re given the mass (80g) but must determine the number of moles of gas. We could use the Ideal Gas Law but it is more convenient to remember that 1 mole of a gas has a volume of 22.4L at STP (1atm and 273K). The conditions in this question are 10atm and 273K. Pressure and volume are inversely proportional so 10 times the pressure will result in the volume of 1 mole of gas being 1/10 the value at STP. Effectively this means that 22.4L would be 10 moles of gas under these conditions. 11.2L is therefore 5 moles of gas.
m/n = (80g) / (5moles) = 16g/mole
The only gas listed with a molar mass of 16g/mole is CH4.
Nitrogen, oxygen, and hydrogen gas are present in a sealed container. If the partial pressure of nitrogen is 300 torr, the partial pressure of oxygen is 200 torr, and the partial pressure of hydrogen is 150 torr, then what is the total pressure?
a. 50 torr
b. 250 torr
c. 350 torr
d. 650 torr
e. 1300 torr
d. 650
idk. This one seems pretty straight-forward
Two moles of helium, seven moles of neon, and one mole of argon are present in a vessel whose total pressure is 500torr. What are the partial pressures of helium, neon and argon respectively?
a. 100torr, 350torr, 50torr
b. 50torr, 300torr, 150torr
c. 150torr, 300torr, 50torr
d. 200torr, 350torr, 50torr
e. 100torr, 450torr, 50torr
a.
I made a misstep here and converted to grams. Don’t do that. Grams have nothing to do with it. Use the moles. Recognize that you have 10 total moles. Break those down into percentages. And then find out what percentage of each comes out of the 500 torr
example: neon is 20% so 20% of 500 is 100 torr
A 10L rigid vessel is filled with 2g He, 30g Ne, and 120g Ar to a total pressure of 2atm. What is the partial pressure of He in the vessel?
a. 0.01atm
b. 0.1atm
c. 0.2atm
d. 0.4atm
e. 0.75atm
c. 0.2 atm
(the 10L vessel is arbitrary info that’s supposed to throw you off)
This one isn’t too bad. The percentage of a percentage part messed with me a little but you can think your way through it, just don’t be hasty before selecting an answer. First convert to moles to see that you have 5 total moles. You can see that He makes up 10% of these 5 moles. So what is 10% of 2 atm?
A pin prick in a balloon filled with CH4 and SO2 allows the gases to slowly escape. Which gas escapes more quickly and how many times more quickly?
hint: the lighter gas will escape more quickly
a. SO2 escapes 2 times more quickly
b. SO2 escapes 16 times more quickly
c. CH4 escapes 2 times more quickly
d. CH4 escapes 4 times more quickly
e. CH4 escapes 16 times more quickly
c.
These next two problems seem harder than they are. If you don’t care about how the formula is derived (and right now, I don’t) just ignore all the noise and take the molar weight of eacah molecule. Assemble those molar weights into a fraction and square root both the demoninator and numerator. If you do this twice with (swapping out which molecule is on top and which is on bottom) you’ll probably get two different answers. e.g. 3 and 1/3 or 2 and 1/2
This means that the lighter molecule will escape 3 times faster or 2 times faster (given the respective examples) and the heavier molecule will escape at only 1/3 of the speed or 1/2 the speed of the lighter one
A pin prick in a balloon filled with He and Ar allows the gases to slowly escape. Which gas escapes more quickly and how many times more quickly?
hint: the lighter gas will escape more quickly
a. Helium escapes 3.2 times more quickly
b. Helium escapes 10 times more quickly
c. Helium escapes 100 times more quickly
d. Argon escapes 10 times more quickly
e. Argon escapes 100 times more quickly
a.
These next two problems seem harder than they are. If you don’t care about how the formula is derived (and right now, I don’t) just ignore all the noise and take the molar weight of eacah molecule. Assemble those molar weights into a fraction and square root both the demoninator and numerator. If you do this twice with (swapping out which molecule is on top and which is on bottom) you’ll probably get two different answers. e.g. 3 and 1/3 or 2 and 1/2
This means that the lighter molecule will escape 3 times faster or 2 times faster (given the respective examples) and the heavier molecule will escape at only 1/3 of the speed or 1/2 the speed of the lighter one
