**Practice Problem 13**

Calculate the volume of H_{2} gas at 25^{o}C
and 1.00 atm that will collect at the cathode when an aqueous
solution of Na_{2}SO_{4} is electrolyzed for 2.00
hours with a 10.0-amp current.

**Solution**

We start by calculating the amount of electrical charge that passes through the solution.

We then calculate the number of moles of electrons that carry this charge.

The balanced equation for the reaction that produces H_{2}
gas at the cathode indicates that we get a mole of H_{2}
gas for every two moles of electrons.

Cathode (-): |
2 H_{2}O + 2 e^{-} H_{2} + 2 OH^{-} |

We therefore get one mole of H_{2} gas at the cathode
for every two moles of electrons that flow through the cell.

We now have the information we need to calculate the volume of the gas produced in this reaction.