Math Problem Statement

A solution NH3 that contains 75 mL of 0.044 M ammonia, NH3, is titrated with 0.087 M HCl. The Kb of ammonia is 1.8 ✕ 10−5. (a) What volume of 0.087 M HCl would be added to reach the equivalence point? Give the volume in mL. mL

(b) At the equivalence point, what is the pH of the solution? (Assume that volumes are additive.)

Solution

We are dealing with a titration problem involving ammonia (NH3\text{NH}_3) as the weak base and hydrochloric acid (HCl) as the strong acid. Let's break the problem down into two parts:

Part (a) - Volume of HCl to reach the equivalence point:

The equivalence point in a titration is reached when the moles of acid equal the moles of base. First, calculate the moles of ammonia in the initial solution.

  1. Moles of NH₃: Moles of NH₃=MNH₃×VNH₃=0.044M×0.075L=0.0033mol\text{Moles of NH₃} = M_{\text{NH₃}} \times V_{\text{NH₃}} = 0.044 \, M \times 0.075 \, L = 0.0033 \, \text{mol}

    The moles of HCl needed to reach the equivalence point will be the same because the reaction between NH₃ and HCl proceeds in a 1:1 molar ratio.

  2. Volume of HCl:

    The moles of HCl can be calculated using the molarity and volume of HCl: Volume of HCl=Moles of HClMHCl=0.0033mol0.087M=0.03793L=37.93mL\text{Volume of HCl} = \frac{\text{Moles of HCl}}{M_{\text{HCl}}} = \frac{0.0033 \, \text{mol}}{0.087 \, M} = 0.03793 \, L = 37.93 \, \text{mL}

Thus, the volume of 0.087M0.087 \, M HCl required to reach the equivalence point is 37.93 mL.

Part (b) - pH at the equivalence point:

At the equivalence point, all the ammonia (NH3\text{NH}_3) has reacted with HCl to form ammonium ions (NH4+\text{NH}_4^+). The pH is determined by the hydrolysis of NH4+\text{NH}_4^+, which acts as a weak acid.

  1. Concentration of NH4+\text{NH}_4^+ after the equivalence point:

    The total volume after adding the acid is: Vtotal=VNH3+VHCl=75mL+37.93mL=112.93mL=0.11293LV_{\text{total}} = V_{\text{NH}_3} + V_{\text{HCl}} = 75 \, \text{mL} + 37.93 \, \text{mL} = 112.93 \, \text{mL} = 0.11293 \, \text{L}

    The concentration of NH4+\text{NH}_4^+ is then: [NH4+]=0.0033mol0.11293L=0.0292M[\text{NH}_4^+] = \frac{0.0033 \, \text{mol}}{0.11293 \, \text{L}} = 0.0292 \, M

  2. Hydrolysis of NH4+\text{NH}_4^+:

    The reaction of NH4+\text{NH}_4^+ with water is: NH4++H2ONH3+H3O+\text{NH}_4^+ + H_2O \rightleftharpoons \text{NH}_3 + H_3O^+

    The equilibrium constant for this reaction is related to the base dissociation constant KbK_b of ammonia by: Ka=KwKb=1.0×10141.8×105=5.56×1010K_a = \frac{K_w}{K_b} = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} = 5.56 \times 10^{-10}

  3. Calculation of H+\text{H}^+ concentration:

    Set up the expression for the ionization of NH4+\text{NH}_4^+ to find the concentration of H+\text{H}^+: [ K_a = \frac{[\text{H}^+][\text{NH}_3]}{[\text{NH}_4^+]} ] Assume [H+]=x[\text{H}^+] = x and [NH4+]=0.0292M[\text{NH}_4^+] = 0.0292 \, M, we get: 5.56×1010=x20.02925.56 \times 10^{-10} = \frac{x^2}{0.0292} Solving for xx, we find: x2=(5.56×1010)×(0.0292)=1.622×1011x^2 = (5.56 \times 10^{-10}) \times (0.0292) = 1.622 \times 10^{-11} x=1.622×1011=1.273×106Mx = \sqrt{1.622 \times 10^{-11}} = 1.273 \times 10^{-6} \, M

    Therefore, [H+]=1.273×106M[\text{H}^+] = 1.273 \times 10^{-6} \, M.

  4. pH Calculation: pH=log[H+]=log(1.273×106)=5.90\text{pH} = -\log[\text{H}^+] = -\log(1.273 \times 10^{-6}) = 5.90

Thus, the pH at the equivalence point is 5.90.


Do you have any questions or want further details on any of these calculations?

Here are 5 related questions for you to think about:

  1. How would the pH change if a stronger base than ammonia were used?
  2. How does the volume of HCl required change with increasing ammonia concentration?
  3. What happens to the pH of the solution if an excess of HCl is added beyond the equivalence point?
  4. How would the titration curve for this reaction look?
  5. How does the value of KbK_b influence the pH at the equivalence point?

Tip: In weak base-strong acid titrations, the equivalence point pH is always acidic due to the hydrolysis of the weak base's conjugate acid.

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Math Problem Analysis

Mathematical Concepts

Titration
Stoichiometry
pH Calculations
Equilibrium
Weak Base Hydrolysis

Formulas

Moles of NH₃ = Molarity × Volume
Volume of HCl = Moles of NH₃ / Molarity of HCl
pH = -log[H⁺]
Ka = Kw / Kb
Hydrolysis equilibrium: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

Theorems

Law of Mass Action
Equivalence Point in Titrations
Relationship between Kb and Ka

Suitable Grade Level

College-level Chemistry