# HELP A problem on quantitative aspects of electrolysis

#### Kudo Shinichi

HELP!!!A problem on quantitative aspects of electrolysis

1. The problem statement, all variables and given/known data
A quantity of electric charge brings about the deposition of 3.34 g Cu at a cathode during the electrolysis of a solution containing Cu2+(aq).
What volume of H2(g) measured at 27.8oC and 767 mmHg, would be produced by this same quantity of electric charge in the reduction of H+(aq) at a cathode?

3. The attempt at a solution
I don't really know the formula for solving the volume in the electrolysis.
The way I approached this problem is by using the ideal gas law(but I don't think it is the right way):
PV=nRT
2H++2e--->H2(g)
we are trying to solve for V, so
V=?
P=767mmHg
R=62.36367L·mmHg·K−1·mol−1
n=2
T=300.95K
plug the numbers into the equation and I got V=48.94L

Can someone please help me with it? Is it the right way for solving the problem? thank you very much.

#### alxm

Re: HELP!!!A problem on quantitative aspects of electrolysis

Well, you got the part about using the ideal gas law to get the volume.
But how much gas did you create? n is the number of moles of gas.

First, how many moles of copper did you produce?
How many moles of electrons does that correspond to?
How many moles of hydrogen gas do you get per mol of electrons?

#### Kudo Shinichi

Re: HELP!!!A problem on quantitative aspects of electrolysis

Well, you got the part about using the ideal gas law to get the volume.
But how much gas did you create? n is the number of moles of gas.

First, how many moles of copper did you produce?
How many moles of electrons does that correspond to?
How many moles of hydrogen gas do you get per mol of electrons?
there are 0.052 moles of copper
0.103 moles of electrons (2 electrons transferred in copper)

I am not really sure about the moles of hydrogen gas but since there are 2 moles of electrons and 1 moles of hydrogen gas, I think that the moles of hydrogen gas is 1/2 of the moles of electrons which is 0.0515
is it right? and do i just plug the moles of hydrogen gas into the ideal gas equation?

#### alxm

Re: HELP!!!A problem on quantitative aspects of electrolysis

there are 0.052 moles of copper
0.103 moles of electrons (2 electrons transferred in copper)

I am not really sure about the moles of hydrogen gas but since there are 2 moles of electrons and 1 moles of hydrogen gas, I think that the moles of hydrogen gas is 1/2 of the moles of electrons which is 0.0515
is it right? and do i just plug the moles of hydrogen gas into the ideal gas equation?
Right, two electrons are used in both reactions so nCu = nH2.
And you put that into the ideal gas equation to get the volume.

#### Borek

Mentor
Re: HELP!!!A problem on quantitative aspects of electrolysis

Kudo, read about Faraday's law of electrolysis. It won't give different result, as it is basically identical to what you did, it is just a systematic way of doing this type of questions.

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