Key Equations
\[Q = I \times t\]
Q = charge (C), I = current (A), t = time (s)
\[n = \frac{Q}{zF}\]n = moles of substance, z = electrons per ion, F = 96485 C mol⁻¹
Step-by-Step Method
- Calculate charge: Q = I × t
- Calculate moles of electrons: n(e⁻) = Q / F
- Use stoichiometry (z) to find moles of substance
- Calculate mass: m = n × M
Worked Example
Q: A current of 2.50 A is passed through CuSO₄(aq) for 30.0 minutes. Calculate the mass of copper deposited at the cathode.
Step 1: \(Q = 2.50 \times (30.0 \times 60) = 4500 \text{ C}\)
Step 2: Cu²⁺ + 2e⁻ → Cu, so z = 2
\(n(\text{Cu}) = \frac{4500}{2 \times 96485} = 0.02332 \text{ mol}\)
Step 3: \(m = 0.02332 \times 63.55 = \textbf{1.48 g}\)
📋 Exam Tips
- Always convert time to seconds before calculating Q
- The value of z depends on the charge on the ion: Cu²⁺(z=2), Al³⁺(z=3), Ag⁺(z=1)
- F = 96485 C mol⁻¹ is given in the IB data booklet