Determine the pH of a 1.2 M solution of ammonia, NH3, with K 1.8 x 105

Chemistry
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ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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**Problem Statement:**

Determine the pH of a 1.2 M solution of ammonia, NH₃, with \( K_b = 1.8 \times 10^{-5} \).

---

For educational purposes, this would involve calculating the hydroxide ion concentration ([OH⁻]) from the base equilibrium equation and using it to find the pOH and subsequently the pH. 

### Steps for Solution:

1. **Write the balanced equation for ammonia in water:**

   \[
   \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-
   \]

2. **Write the expression for the base dissociation constant (\( K_b \)):**

   \[
   K_b = \frac{[\text{NH}_4^+][\text{OH}^-]}{[\text{NH}_3]}
   \]

3. **Make assumptions for initial concentrations and changes:**

   - Initial \([\text{NH}_3] = 1.2 \, \text{M}\)
   - Changes in concentration are symbolized as \(-x\) for \([\text{NH}_3]\) and \(+x\) for \([\text{NH}_4^+]\) and \([\text{OH}^-]\).

4. **Solve for \(x\):**

   - Substitute into \(K_b\) expression:

     \[
     1.8 \times 10^{-5} = \frac{x \cdot x}{1.2 - x} 
     \]

     - Assume \(1.2 - x \approx 1.2\) due to small value of \(K_b\):

     \[
     1.8 \times 10^{-5} = \frac{x^2}{1.2}
     \]

     \[
     x^2 = 1.8 \times 10^{-5} \times 1.2
     \]

     \[
     x^2 = 2.16 \times 10^{-5}
     \]

     \[
     x = \sqrt{2.16 \times 10^{-5}} \approx 0.00464
     \]

5. **Calculate pOH and pH:**

   - pOH = \(-
Transcribed Image Text:**Problem Statement:** Determine the pH of a 1.2 M solution of ammonia, NH₃, with \( K_b = 1.8 \times 10^{-5} \). --- For educational purposes, this would involve calculating the hydroxide ion concentration ([OH⁻]) from the base equilibrium equation and using it to find the pOH and subsequently the pH. ### Steps for Solution: 1. **Write the balanced equation for ammonia in water:** \[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \] 2. **Write the expression for the base dissociation constant (\( K_b \)):** \[ K_b = \frac{[\text{NH}_4^+][\text{OH}^-]}{[\text{NH}_3]} \] 3. **Make assumptions for initial concentrations and changes:** - Initial \([\text{NH}_3] = 1.2 \, \text{M}\) - Changes in concentration are symbolized as \(-x\) for \([\text{NH}_3]\) and \(+x\) for \([\text{NH}_4^+]\) and \([\text{OH}^-]\). 4. **Solve for \(x\):** - Substitute into \(K_b\) expression: \[ 1.8 \times 10^{-5} = \frac{x \cdot x}{1.2 - x} \] - Assume \(1.2 - x \approx 1.2\) due to small value of \(K_b\): \[ 1.8 \times 10^{-5} = \frac{x^2}{1.2} \] \[ x^2 = 1.8 \times 10^{-5} \times 1.2 \] \[ x^2 = 2.16 \times 10^{-5} \] \[ x = \sqrt{2.16 \times 10^{-5}} \approx 0.00464 \] 5. **Calculate pOH and pH:** - pOH = \(-
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