Apply Faraday's laws to calculate masses and volumes produced in electrolysis.

What this dot point is asking

You must perform quantitative electrolysis calculations linking charge, moles of electrons and mass or volume of product.

Charge and the Faraday constant

The charge passed through a cell is:

One mole of electrons carries a charge equal to the Faraday constant, $F = 96\,500\ \text{C mol}^{-1}$. So the moles of electrons passed is $n(\text{e}^-) = Q / F$.

Linking electrons to product

The electrode half-equation tells you how many electrons are needed per mole of product. For example, depositing copper requires $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$, so 2 moles of electrons give 1 mole of copper. The general routine is:

• Find charge $Q = It$.
• Find moles of electrons $n(\text{e}^-) = Q / F$.
• Use the half-equation ratio to find moles of product.
• Convert moles of product to mass ($m = nM$) or gas volume.

Gas volumes

If the product is a gas, convert moles to volume using the molar gas volume at the stated conditions, or the ideal gas equation. The same electron bookkeeping applies; just watch the electrons per mole of gas, which is often more than for a metal.

In the exam, convert time to seconds, compute $Q = It$, divide by the Faraday constant for moles of electrons, then apply the half-equation ratio before converting to mass or volume.