To determine the equivalence point of the titration between ammonia (NH₃) and hydrochloric acid (HCl), we first need to understand what occurs at the equivalence point.
-
Ammonia (NH₃) is a weak base and when titrated with hydrochloric acid (HCl), a strong acid, they react to form ammonium chloride (NH₄Cl): \[ \text{NH}_3(aq) + \text{HCl}(aq) \rightarrow \text{NH}_4^+(aq) + \text{Cl}^-(aq) \]
-
At the equivalence point, all of the NH₃ will have reacted with an equivalent amount of HCl.
-
To find the equivalence point, we determine the moles of each solution involved:
Let \( V_1 \) be the volume of NH₃ solution and \( V_2 \) be the volume of HCl solution used. The number of moles of each reactant can be expressed as:
- Moles of NH₃ = \( 0.3 , \text{M} \times V_1 \)
- Moles of HCl = \( 0.2 , \text{M} \times V_2 \)
At the equivalence point, the moles of NH₃ will equal the moles of HCl: \[ 0.3 , V_1 = 0.2 , V_2 \] \[ \frac{V_1}{V_2} = \frac{0.2}{0.3} = \frac{2}{3} \]
This shows that for every 2 mL of NH₃, we need 3 mL of HCl to reach the equivalence point.
-
Now, we look at the pH at the equivalence point. At this point, only NH₄⁺ (the conjugate acid of NH₃) and Cl⁻ ions will be present in the solution. NH₄⁺ can donate a proton to water, acting as a weak acid: \[ \text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{H}_3\text{O}^+ \]
-
To find the pH at the equivalence point, we must calculate the concentration of \( \text{NH}_4^+ \) after titration, then use \( K_a \) for the dissociation of NH₄⁺. The \( K_a \) can be calculated from the \( K_b \) of NH₃ which is approximately \( 1.8 \times 10^{-5} \): \[ K_a = \frac{K_w}{K_b} = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} \approx 5.56 \times 10^{-10} \]
-
The pH can be found from the concentration of NH₄⁺ and the equilibrium expression for \( K_a \): \[ K_a = \frac{[\text{NH}_3][\text{H}_3\text{O}^+]}{[\text{NH}_4^+]} \]
Assuming the concentration of \( \text{NH}_4^+ \) at the equivalence point is \( C \), then we can derive the pH. The pH is not straightforward to calculate because it requires solving for equilibrium concentrations, but typically for dilute solutions, you can estimate.
- Typically, the pH at the equivalence point of a weak base (NH₃) with a strong acid (HCl) will be below 7, likely in the range of 5-6 due to the formation of NH₄⁺.
In summary, at the equivalence point, the solution is composed of NH₄Cl. The general pH at the equivalence point of this reaction is likely between 5-6, but precise calculation would require detailed information on the volumes used, and potentially some further calculations to determine the concentrations and the resulting hydronium concentration.