[formaldehyde23]

I have four chem questions here, and I'd really appreciate if someone could help me out. Along with the questions, I'm also posting how far I am, so maybe you can build on that? Thanks!

1) Through a series of enzymatic steps, carbon dioxide and water undergo photosynthesis to produce glucose and oxygen according to the equation
6CO₂(g) + 6H₂O  C₆H₁₂O₆ + 6O₂.

Given that the partial pressure of carbon dioxide in the troposphere is 0.26 Tor and the temperature is 25 degrees Celsius, calculate the volume of air needed to produce 10.0 g of glucose.

Sol: I converted the glucose into moles and then used basic stoich. to get moles of Carbon dioxide (.33 mol). I also know the mol fraction of carbon dioxide in air (3.19*10^-4). So, from here, I can calculate the pressure of air. But, what do I do next? How can I use this pressure to calculate my volume of air?

2) A 1.00 L flask contains nitrogen gas at a temperature of 15 degrees Celsius and a pressure of 0.50 bar. 0.10 mol O₂ is added to the flask and allowed to the mix. Then a stopcock is opened to allow 0.20 mol of molecules to escape. What is the partial pressure of oxygen in the final mixture?

Sol: I'm just confused here; I calculated the moles of nitrogen gas (.0211 mol). But, there's a total of .1211 mol of gas with nitrogen and oxygen. How can 0.20 mol of gas escape?

3) In this question, I have four different flasks. They're composed of different gases, but they have the same volume and temperature. One's prepared with Helium atoms, the second with chlorine molecules, the third with argon atoms, and the fourth with ammonia molecules. Would all flasks have the same pressure?

Sol: My thought process here was that the smaller the  molecules, the greater the pressure (more collisions?), but part of me thinks that all four containers would have the same pressure. Does particle size affect pressure? I would think not, as pressure is impacted by concentration, temperature, etc...

4) A finely powdered solid sample of an osmium oxide (which melts at 40 deg C and boils at 130 deg C)  with a mass of 1.509 g is placed into a cylinder with a movable piston that can expand against the atmospheric pressure of 745 Torr. Assume that the amount of residual air initially present in the cylinder is negligible. When the sample is heated to 200 degrees Celsius, it is completely vaporized and the volume of the cylinder expands by 235 mL. What is the molar mass of the oxide? Assuming that the oxide is OsO_x, find the value of x.

Sol: I have absolutely no idea how to do this problem, how to even approach it....

[Borek]

Sol: I converted the glucose into moles and then used basic stoich. to get moles of Carbon dioxide (.33 mol). I also know the mol fraction of carbon dioxide in air (3.19*10^-4).

How many "moles of air" required for 0.33 moles of CO₂ given the molar fraction?

[formaldehyde23]

thanks Borek!

I just didn't think to use my mol fraction to calculate the total moles of the air.
After using V = nRT/P, I got 2.4*10^4 L, which is correct.

Any ideas how to solve the other 3?

[billnotgatez]

I was wondering about number 3 as well
If we can assume the temperature and volume can be of our choosing
Then we could make all the volumes 22.414 liters
Then we could make the temperature 273.15 kelvins (0C)
And if we could assume Ideal Gas
Do you have any thoughts

[Borek]

3rd is unclear. If we can treat gases as ideal (it mainly depends on the average distance between molecules and their size) pressures are identical. If we can't, pressures are different.

Correct answer depends on the level. At HS "identical" is probably the expected answer.

2nd has no answer, the is an error in the data (which leads to a nonsense that you already mentioned).

4th: how many moles of oxygen? What is its mass? How many moles of Os in the original sample?

[formaldehyde23]

With some help from a friend, I've got number 3

All 4 containers have the same pressure.
We know that impulse equals the change in momentum:
FdT = mdV.
Larger particles have a greater mass but a smaller velocity; smaller particles have a smaller mass but greater velocity. So, both types of particles have the same momentums. Therefore, their impulse is the same...
Since their force is the same, their pressure is the same.

Therefore, pressure is not correlated with particle size, and all 4 containers have the same pressure.

[Borek]

Therefore, pressure is not correlated with particle size, and all 4 containers have the same pressure.

This is only one of two possible answers, as explained in my previous post.

And you don't need a long, winded explanation, all you need is the ideal gas equation and Avogadro's law.

[billnotgatez]

I have the same answer (all 4 pressures same) for number 3 but my reasoning was entirely mathematical manipulation of Ideal Gas Law.

[billnotgatez]

@formaldehyde23
Did you work on number 4

Did they eventually give you what they wanted the answer to be in number 2 -- so we can back into the error in the question

[formaldehyde23]

@billnotgatez

All of these questions are from Atkins: Chemical Principles - The Quest for Insight.

For number 2, I'm assuming that it's a mistake on the authors' part. So, let's say that 0.10 moles of gas were extracted from the container. How would I approach the problem then?

Could I just set up mathematically: Since there are .0211 moles of nitrogen and .1 moles of oxygen, then..
(.0211 + .1)x = .1. Solve for x and find the number of moles of oxygen left.
Then, I can just calculate partial pressure easily...
Is that a flawed way of thinking about this?

For number 4, I'm just lost on how to approach it... Borek thinks that it might be missing details. Do you have any ideas?

[Borek]

(.0211 + .1)x = .1. Solve for x and find the number of moles of oxygen left.

That would be correct.
 

For number 4, I'm just lost on how to approach it... Borek thinks that it might be missing details. Do you have any ideas?

No, I don't think it misses any details.

Edit: I just realized I misread the problem, thinking oxide decomposed, while it just boiled away. Still, it can be easily solved. Just calculate how many moles of the oxide are present.

[billnotgatez]

@formaldehyde23
The problem does not directly state "ignore any effects that the piston may have on the pressure inside the cylinder", but I think it is implied. Assuming that can you calculate the moles?

[formaldehyde23]

Yea, makes sense. n = PV/RT and then we can calculate MM easily...

I was just confused about the melting and boiling point data when I first approached the problem. Even though 200 degrees surpasses the boiling point, I guess it doesn't affect our calculations...

[billnotgatez]

The 200C insures that the material is totally a gas. If they had mentioned a decomposition temperature you would have needed to deal with that as well.
Do you want to show the computations?
I still think the [OsOx, find the value of x] adds some interest.
I assume you do not have access to an answer key for Atkins: Chemical Principles - The Quest for Insight.