The emf of cell Tl(0.001\ M)Tl+(0.01\ M)Cu2+Cu is 0.83 V at 298 K. It could be increased by:

Strategy: Apply Le Chatelier's principle to the Nernst Equation. The cell EMF depends on the ratio of species in the reaction quotient ($Q$). Increasing reactants or decreasing products shifts the equilibrium to favor the forward reaction (increasing EMF).

Step 1: Write the cell reaction From the notation $\ce{Tl | Tl^+ || Cu^{2+} | Cu}$:

• Anode: $\ce{Tl \rightarrow Tl^+ + e^-}$
• Cathode: $\ce{Cu^{2+} + 2e^- \rightarrow Cu}$
• Net Reaction: $\ce{2Tl + Cu^{2+} \rightarrow 2Tl^+ + Cu}$

Step 2: Apply the Nernst Equation $E_{\text{cell}} = E^\circ_{\text{cell}} - \frac{0.059}{2} \log \frac{[\ce{Tl^+}]^2}{[\ce{Cu^{2+}}]}$

Step 3: Analyze concentration effects To increase $E_{\text{cell}}$, we must decrease the value of the $\log$ term:

• Increase ${\text[Cu^{2+}]}$ (reactant concentration): This decreases $Q$, thus increasing $E_{\text{cell}}$. ✓
• Decrease ${\text[Tl^+]}$ (product concentration): This also increases $E_{\text{cell}}$. ✓

Comparing with the options, increasing $\ce{Cu^{2+}}$ is the valid choi\ce.

$\boxed{\text{Answer: (b) increasing concentration of } \ce{Cu^{2+}} \text{ ions}}$