Calculate the molarity of solution A using the provided redox titration equations.

$$\text{Moles of S}_2\text{O}_3^{2-} = \text{Concentration} \times \text{Volume}$$ $$\text{Moles of S}_2\text{O}_3^{2-} = 0.025 \text{ mol dm}^{-3} \times \frac{15.65}{1000} \text{ dm}^3$$ $$\text{Moles of S}_2\text{O}_3^{2-} = 0.00039125 \text{ mol}$$

From equation (2), the mole ratio of $\text{S}_2\text{O}_3^{2-}$ to $\text{I}_2$ is $2:1$.

$$\text{Moles of I}_2 = \frac{\text{Moles of S}_2\text{O}_3^{2-}}{2}$$ $$\text{Moles of I}_2 = \frac{0.00039125 \text{ mol}}{2}$$ $$\text{Moles of I}_2 = 0.000195625 \text{ mol}$$

From equation (1), the mole ratio of $\text{Cr}_2\text{O}_7^{2-}$ to $\text{I}_2$ is $1:3$.

$$\text{Moles of Cr}_2\text{O}_7^{2-} = \frac{\text{Moles of I}_2}{3}$$ $$\text{Moles of Cr}_2\text{O}_7^{2-} = \frac{0.000195625 \text{ mol}}{3}$$ $$\text{Moles of Cr}_2\text{O}_7^{2-} = 0.0000652083 \text{ mol}$$

The volume of solution A used was $25.0 \text{ cm}^3$ ($0.025 \text{ dm}^3$).

$$\text{Molarity of A} = \frac{\text{Moles of Cr}_2\text{O}_7^{2-}}{\text{Volume of A}}$$ $$\text{Molarity of A} = \frac{0.0000652083 \text{ mol}}{0.025 \text{ dm}^3}$$ $$\text{Molarity of A} = 0.00260833 \text{ mol dm}^{-3}$$

The molarity of solution A is $\boxed{0.00261 \text{ M}}$ (to 3 significant figures).

1) (ii) Is the reaction occurring in table 2 exothermic or endothermic? Explain.
The reaction is endothermic.
The temperature of the water decreased from $27.0^\circ\text{C}$ to $21.5^\circ\text{C}$. This indicates that the reaction absorbed heat from the surroundings (the water), causing the temperature of the water to drop.

Question 2

a) (i) Carry out a flame test on a solid sample of E.

• Observations: Lilac flame
• Inferences: The presence of potassium ions ($\text{K}^+$). (A lilac flame is characteristic of potassium.)

a) (ii) To a solid portion of E in a dry test tube, add $1 \text{ cm}^3$ of conc. $\text{H}_2\text{SO}_4$, test for any gas evolved with red and blue litmus paper.

• Observations: Gas turns blue litmus paper red; red litmus paper shows no observable change. This indicates an acidic gas is evolved.
• Inferences: The evolution of an acidic gas (likely brown fumes of $\text{NO}_2$) when a solid reacts with concentrated sulfuric acid suggests the presence of nitrite ions ($\text{NO}_2^-$). (Nitrites react with conc. $\text{H}_2\text{SO}_4$ to produce acidic brown fumes of nitrogen dioxide.)