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Annonn Annonn
wrote...
Posts: 213
Rep: 1 0
2 years ago

1. Calculate the number of moles of hydrogen produced. (each trial)
Given:
- Gas Constant (R) =  0.08205 (L*Atm)/(mol*K); units on the gas constant are Liters, Atmospheres, & Kelvins
- I'm told to convert my experimental values to the above's units to calculate properly
- Calculation of the number of coulombs passed: Coulombs = Amperes × Seconds

2. Calculate the  Faraday’s Constant. (each trial)
Given:
- Determine the number of moles of electrons passed using the reaction stoichiometry
- Determine the number of moles of hydrogen gas produced
- F = couloumbs/mole-
- I think this is the stoichiometry for my lab: 2H+(aq) + 2e- → H2(g) or H+(aq) + 1e- → ½ H2(g)

I also need to provide:
- Average (I can do this)
- Percent error
- Standard deviation (I can do this)
- Root mean squared
- Deviation for your experimental values of Faraday’s constant

3. Calculate the  Avogadro’s Number. (each trial)
Given:
- Charge on a single electron= 1.60 X 10-19 C
-I'm told to use my calculated value of the total charge passed during the reaction to calculate the total number of electrons consumed in the reaction at the cathode.

I also need to provide:
- Average (I can do this)
- Percent error
- Standard deviation (I can do this)
- Root mean squared
- Deviation for your experimental values of Avogadro’s number


I would love it if someone helped steer me in the right direction to calculate what I am required above. All three trials don't need to be used to help me, but if someone can use just one of the trials for my calculations, I can replicate that across the other trials. I also don't need someone to calculate the answers, just help me formulate how to get there if that makes sense. And if willing, I would love to go over my calculations with someone to double-check my work.
I can provide my lab manual if anyone wants to see it. Any help is appreciated deeply!
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Replies
wrote...
Educator
2 years ago
1. Calculate the number of moles of hydrogen produced. (each trial) Given: - Gas Constant (R) =  0.08205 (L*Atm)/(mol*K); units on the gas constant are Liters, Atmospheres, & Kelvins - I'm told to convert my experimental values to the above's units to calculate properly - Calculation of the number of coulombs passed: Coulombs = Amperes × Seconds

Hi Annonn

For the first question, I found a sample calculation that might help you. Take, for example, the following table:



A model calculation given this table is shown below:





Now, assuming we are still using the initial table of values above, we get the following moles of hydrogen for each run:



Quote
2. Calculate the  Faraday’s Constant. (each trial)





Annonn Author
wrote...
2 years ago
Hey! Thank you so much for helping out, I know this might be much to ask for, but would you mind helping me understand that last part, 3. Calculate the Avogadro’s Number. I'm not sure what formula(s) to use  Smiling Face with Open Mouth and Cold Sweat

So far I've used your sample example for the Faraday and Moles of Hydrogen part, it was extremely useful!!
wrote...
Educator
2 years ago
I'm not really sure, Annonn

From the definition of Faraday, it is the total charge of 1 mole of electrons.

1 mole of electrons = Avogadro's number of electrons.

In theory, 1 Faraday = 96,500 coulomb, and the charge on the electron is 1.60 x 10-19 C. Using what you found for Faraday's number, say: 96,300

=> 96,300 = NA x 1.60 x 10^-19

=> NA = 96,300 / 1.60 x 10^-19

=> NA = 6.02 x 10^23

Apart from this, I'm not really sure...
Annonn Author
wrote...
2 years ago
Shoot, well thank you for trying!! I'll see if I can ask the lab instructor for any help (they're not really reliable, hence why I use Biology forums)
wrote...
Educator
2 years ago
Shoot, well thank you for trying!! I'll see if I can ask the lab instructor for any help (they're not really reliable, hence why I use Biology forums)

Let us know what you find
Annonn Author
wrote...
2 years ago
Ok, the lab instructor messaged me back and said this:
"The final act. To obtain Avogadro's number, we should divide our number of Hydrogen molecules by moles of Hydrogen molecules. Your answer should be about 6 x 10^23."

Honestly, this doesn't help me and so I went through the lab manual and I'll paste its directions word-for-word for the last step:

"Recall that the charge on a single electron is 1.60 X 10-19 C. Use this value, and your calculated value of the total charge passed during the reaction to calculate the total number of electrons consumed in the reaction at the cathode.

Consider how knowing the number of electrons consumed in the reaction and the reaction stoichiometry will allow you to calculate the number of molecules of H2 gas produced, then do so for each reaction. Show your calculations.

The previous calculations have allowed you to determine values for the number of moles of hydrogen gas produced, and the number of molecules of hydrogen gas produced. Use these values to calculate the value of Avogadro’s number for each reaction, keeping in mind that Avogadro’s number is the number of molecules in a mole of substance.

Calculate the average, percent error, standard deviation, and root mean squared deviation for your experimental values of Avogadro’s number. Show your calculations in your lab notebook."


I'm still confused, hopefully this clear things up?
wrote...
Valued Member
2 years ago
So far what has been calculated are the moles of hydrogen produced for each run using n = PV/RT, the coulombs using C = amperes * seconds, the number of moles of e- for the reaction 2H+ + 2e- -> H2 for each run, and Faraday's number using the formula F = coulombs / moles of e-.

That being said, your instructor writes: "To obtain Avogadro's number, we should divide our number of Hydrogen molecules by moles of Hydrogen molecules." I can see why your confused, because it doesn't make sense to me either Undecided

Avogadro's number is calculated by taking the number or electrons / moles of electrons. So take what you get for "C = amperes * seconds" and divide it by the moles of electrons.

For example, let's say you get 130 C when you multiply C = amperes * seconds.

130 C * [1 electron / (1.60 * 10-19 C)] = # of electrons => 8.125 * 1020

Now we divide this by the number of moles of e-. Say it is: 13.9 * 10-4

8.125 * 1020 / 13.9 * 10-4 = 5.85 * 1023

Does that help? If you need more clarification, please see the attachment Smiling Face with Halo
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Annonn Author
wrote...
2 years ago
Thank you for replying, I was just doing some calculations based on what my lab instructor said. So I am scrapping all that and using your wonderful advice. It makes much better sense to me, so I'll be basing my calculations on your examples. Slight Smile
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