Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (b) NH3 titrated with HCl.

Verified step by step guidance

1. Identify the nature of the reactants: NH3 is a weak base and HCl is a strong acid.
2. Understand that the equivalence point in a titration occurs when the moles of acid equal the moles of base.
3. Recognize that at the equivalence point, the solution will primarily contain the conjugate acid of the weak base, NH4+.
4. Recall that NH4+ is a weak acid, which will slightly dissociate in water to produce H+ ions, lowering the pH.
5. Conclude that the equivalence point for the titration of NH3 with HCl will be below pH 7 due to the presence of NH4+.

Key Concepts

Acid-Base Titration

An acid-base titration is a quantitative analytical method used to determine the concentration of an acid or base in a solution. During the titration, a solution of known concentration (the titrant) is added to a solution of unknown concentration until the reaction reaches its equivalence point, where the amount of acid equals the amount of base. The pH at this point can vary depending on the strengths of the acid and base involved.

Weak Base and Strong Acid

In the titration of a weak base, such as ammonia (NH3), with a strong acid like hydrochloric acid (HCl), the resulting solution at the equivalence point contains the conjugate acid (NH4+) of the weak base. This conjugate acid can donate protons to water, leading to a decrease in pH. Therefore, the pH at the equivalence point will be below 7, indicating an acidic solution.

pH Scale and Equivalence Point

The pH scale measures the acidity or basicity of a solution, with values below 7 indicating acidity, 7 being neutral, and above 7 indicating basicity. The equivalence point in a titration is critical for determining the pH of the resulting solution. For titrations involving a weak base and a strong acid, the pH at the equivalence point will typically be less than 7 due to the formation of a weakly basic conjugate acid.