Lots of people read this, but no one helped.

[Fittleroni]

1. Homework Statement

(a) How many atoms of helium gas fill a balloon of diameter 29.4 cm at 23.5°C and 1.00 atm?
(b) What is the average kinetic energy of the helium atoms?
(c) What is the root-mean-square speed of the helium atoms?


2. Homework Equations

P=2/3[N/V][1/2mv^2]
v=srt(3RT/M)
R=8.31
T=23.5 degrees celcius = 296.5K
M(He)=4.0026g/mol
E(int)=3/2nRT
Avogadros #: 6.0221415 × 10^23

3. The Attempt at a Solution
(a)I don't know how to find the # of molecules. I know I need Avogadros #.
(b)To solve this, don't I need the first one solved?
(c) v=sqrt((3(8.31)(296.5K))/4.0026 g/mol) = 42.97m/s = 0.0429km/s (which is wrong coz I did this part first)

 

[CompuChip]

For a), you should use the ideal gas law, which is not in your list yet
It gives you a connection between the pressure, volume, temperature and number of molecules, three of which are given.
Then for b) you can apply one the first two formulas you quoted, plugging in the number N you found in a).
And I don't understand the title.

 

[Fittleroni]

I had posted this question before, and it was read around 30 times and no one helped me out.

 

[Fittleroni]

Allright, so I did PV/RT=n, and this did not work. I tried multiplying my result times Avogadro's number and that was not right, so I have exhausted all of my tries, and I have still not arrived at the correct answer for (a). Any more advice?

 

[CompuChip]

The ideal gas law can also be written as
[tex]p V = N k_B T,[/tex]
where p is pressure, V is volume, N is number of molecules, T is temperature, and
[tex]k_B \approx 1.38 \times10^{-23} \mathrm{J \cdot K^{-1}}[/tex]
is the Boltzmann constant.

This is much more useful here (though it is exactly the same, as there is the relation R = (Avogadro's #) * (Boltzmann constant)) and it should work. If not, you should give us more information.

As for nobody responding, I apologize, but also note that people here helping other people is a courtesy and not a duty Maybe nobody knew the answer, maybe people were just busy. You may also want to consult https://www.physicsforums.com/showthread.php?t=94379.
By the way, I've asked questions on forums where about 300 people read the thread but nobody responded. So 30 is not that much

 

[hage567]

Allright, so I did PV/RT=n, and this did not work. I tried multiplying my result times Avogadro's number and that was not right, so I have exhausted all of my tries, and I have still not arrived at the correct answer for (a). Any more advice?

Please show details of your calculations. Just saying you didn't get the right answer isn't very helpful, because there are lots of small (or large) mistakes you could be making. We can't tell you what you're doing wrong if you don't show us what you did.

 

[PFStudent]

Hey,

1. Homework Statement

(a) How many atoms of helium gas fill a balloon of diameter 29.4 cm at 23.5°C and 1.00 atm?
(b) What is the average kinetic energy of the helium atoms?
(c) What is the root-mean-square speed of the helium atoms?


2. Homework Equations

P=2/3[N/V][1/2mv^2]
v=srt(3RT/M)
R=8.31
T=23.5 degrees celcius = 296.5K
M(He)=4.0026g/mol
E(int)=3/2nRT
Avogadros #: 6.0221415 × 10^23

3. The Attempt at a Solution
(a)I don't know how to find the # of molecules. I know I need Avogadros #.
(b)To solve this, don't I need the first one solved?
(c) v=sqrt((3(8.31)(296.5K))/4.0026 g/mol) = 42.97m/s = 0.0429km/s (which is wrong coz I did this part first)

Learning LaTeX would have made it easier to understand your calculations. Also, showing more of your work would have helped.

You can refer to the following thread for a somewhat similar problem.

https://www.physicsforums.com/showthread.php?t=178008



-PFStudent