1. **Stating the problem:** We have two questions from the list: - Question 83: Identify which description does NOT correspond to the equivalence point in acid-base titrations. - Question 85: Calculate the concentration of an ammonia solution given volumes and concentration of HCl used in titration. --- 2. **Question 83:** The equivalence point in acid-base titrations is characterized by: - (A) No reagent is in excess. - (B) The amount of $\mathrm{H_3O^+}$ equals the amount of $\mathrm{OH^-}$. - (C) The concentration of acid equals the concentration of base. - (D) Acid and base are in stoichiometric proportion. We need to find which statement is NOT true. **Explanation:** - At equivalence point, the moles of acid and base are equal, but their concentrations may differ because volumes can be different. - So, (C) is false because concentrations do not necessarily equal at equivalence point. **Answer:** (C) --- 3. **Question 85:** Given: - Volume of HCl solution, $V_{HCl} = 37.5$ mL - Concentration of HCl, $C_{HCl} = 0.250$ mol/dm$^3$ - Volume of ammonia solution, $V_{NH_3} = 25.0$ mL - Find concentration of ammonia solution, $C_{NH_3}$ **Reaction:** $$\mathrm{HCl + NH_3 \rightarrow NH_4^+ + Cl^-}$$ Molar ratio is 1:1. **Step 1: Calculate moles of HCl used:** $$n_{HCl} = C_{HCl} \times V_{HCl} = 0.250 \times \frac{37.5}{1000} = 0.009375\,\text{mol}$$ **Step 2: At equivalence, moles of NH$_3$ = moles of HCl:** $$n_{NH_3} = 0.009375\,\text{mol}$$ **Step 3: Calculate concentration of NH$_3$:** $$C_{NH_3} = \frac{n_{NH_3}}{V_{NH_3}} = \frac{0.009375}{\frac{25.0}{1000}} = 0.375\,\text{mol/dm}^3$$ **Answer:** (A) 0.375 mol/dm$^3$ --- **Final answers:** - Question 83: (C) - Question 85: (A) 0.375 mol/dm$^3$