Calculate the pH when 45.0 mL of 0.200 M HBr is mixed with 43.0 mL of 0.400 M CH:NH2 (Kb = 4.4 × 104).

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**Calculate the pH when 45.0 mL of 0.200 M HBr is mixed with 43.0 mL of 0.400 M CH₃NH₂ (Kb = 4.4 × 10⁻⁴).**

This problem involves a reaction between hydrobromic acid (HBr), a strong acid, and methylamine (CH₃NH₂), a weak base. The goal is to determine the pH of the resulting solution after the acid-base reaction.

**Conceptual Steps to Solve the Problem:**

1. **Calculate moles of HBr and CH₃NH₂**:
   - Use the formula: moles = molarity × volume (L).
   
2. **Determine the limiting reactant**:
   - This will be the reactant that is completely consumed in the reaction. 

3. **Calculate the amount of CH₃NH₃⁺ formed**:
   - The reaction between HBr and CH₃NH₂ will form the conjugate acid CH₃NH₃⁺.

4. **Calculate the pH**:
   - If there is an excess of strong acid, use it to find the pH directly.
   - If the base is in excess, calculate the concentration of CH₃NH₃⁺ and use it to determine the pH of the solution considering the hydrolysis of CH₃NH₃⁺.

### Additional Information:
- **Equilibrium Calculations** may be required if there is a significant concentration of the weak base or its conjugate acid.
- **pH determination of weak acid salts** involves using the relationship between Ka and Kb:
  - \( K_w = K_a \times K_b \)
  - \( K_a = \frac{K_w}{K_b} \) for weak base conjugates.

This is an acidity/basicity determination problem that involves stoichiometry, the concept of limiting reactions, and equilibrium. Perfect for chemistry students studying acid-base reactions and pH calculations.
Transcribed Image Text:**Calculate the pH when 45.0 mL of 0.200 M HBr is mixed with 43.0 mL of 0.400 M CH₃NH₂ (Kb = 4.4 × 10⁻⁴).** This problem involves a reaction between hydrobromic acid (HBr), a strong acid, and methylamine (CH₃NH₂), a weak base. The goal is to determine the pH of the resulting solution after the acid-base reaction. **Conceptual Steps to Solve the Problem:** 1. **Calculate moles of HBr and CH₃NH₂**: - Use the formula: moles = molarity × volume (L). 2. **Determine the limiting reactant**: - This will be the reactant that is completely consumed in the reaction. 3. **Calculate the amount of CH₃NH₃⁺ formed**: - The reaction between HBr and CH₃NH₂ will form the conjugate acid CH₃NH₃⁺. 4. **Calculate the pH**: - If there is an excess of strong acid, use it to find the pH directly. - If the base is in excess, calculate the concentration of CH₃NH₃⁺ and use it to determine the pH of the solution considering the hydrolysis of CH₃NH₃⁺. ### Additional Information: - **Equilibrium Calculations** may be required if there is a significant concentration of the weak base or its conjugate acid. - **pH determination of weak acid salts** involves using the relationship between Ka and Kb: - \( K_w = K_a \times K_b \) - \( K_a = \frac{K_w}{K_b} \) for weak base conjugates. This is an acidity/basicity determination problem that involves stoichiometry, the concept of limiting reactions, and equilibrium. Perfect for chemistry students studying acid-base reactions and pH calculations.
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