Answer and Explanation for Question 3
The volume of H₂S needed to neutralize 45 mL of NH₂⁻ is 30 mL. Thus, the correct answer would be B.
To calculate this, we need to use the stoichiometric equation. First, we find the moles of NH2- present in the solution: 45 mL × 0.80 M = 0.036 moles. Then, we need to find the moles of H2S needed to neutralize this, which is equal to the moles of NH2- since the reaction is 1:1. Therefore, we need 0.036 moles of H2S. To convert this to volume, we need to divide by the molarity of the H2S solution: 0.036 mol/1.2 M = 0.03 L, or 30 mL.
Answer and Explanation for Question 4:
The indicator most appropriate for this reaction would be phenolphthalein. Thus, the correct answer would be D.
In this acid-base titration, a solution of HCl is added to an unknown concentration of a base, with the initial pH of the solution being approximately 10. As HCl continues to be added to the mixture, the pH remains relatively unchanged until 50 mL of HCl have been added, at which point the pH of the mixture drops sharply to approximately 3, indicating that the equivalence point has been reached.
Indicators can help us detect the equivalence point of a titration. Phenolphthalein changes color from pink to colorless when the pH of the mixture drops below 8.3, which is what happens in this reaction. Therefore, phenolphthalein would be the most appropriate indicator for this reaction.