How many faraday of electricity is required to produce 10 g of calcium metal (molar mass = 40 $\text{g}\ \text{mol}^{-1}$) from calcium ions?
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Always write down the half-reaction first to see how many electrons are transferred per ion. For Group 2 elements like Ca, Mg, and Ba, it is always 2 Faradays per mole.
Step 1: Write the reaction.
Calcium ions gain electrons to become calcium metal.
\[ \text{Ca}^{2+} + 2e^- \rightarrow \text{Ca} \]
Step 2: Read the electron count.
The equation shows that 1 mole of calcium needs 2 moles of electrons, which is 2 faraday.
Step 3: Find the moles of calcium.
\[ \text{moles} = \frac{\text{mass}}{\text{molar mass}} = \frac{10}{40} = 0.25\ \text{mol} \]
Step 4: Set up the charge needed.
Each mole needs 2 F, so multiply moles by 2.
\[ \text{Faraday} = 0.25 \times 2 \]
Step 5: Do the multiplication.
\[ \text{Faraday} = 0.50\ \text{F} \]
Step 6: Conclusion.
So 0.50 F of electricity is needed. Always write the half-reaction first to see how many electrons each ion needs.
\[ \boxed{0.50\ \text{F (Option 3)}} \]