The hydroxyl ion concentration in a solution having pH value

To find the hydroxyl ion concentration \([\text{OH}^-]\) in a solution when given the pH value, you can use the relationship between pH, pOH, and the ion concentrations of hydrogen ions \([\text{H}^+]\) and hydroxyl ions \([\text{OH}^-]\).

### Steps to Calculate \([\text{OH}^-]\) from pH:

1. **Determine the pOH**:
   - pH and pOH are related by the equation:
     \[
     \text{pH} + \text{pOH} = 14
     \]
   - So, if you know the pH, you can calculate pOH as:
     \[
     \text{pOH} = 14 - \text{pH}
     \]

2. **Calculate the hydroxyl ion concentration \([\text{OH}^-]\)**:
   - pOH is related to the hydroxyl ion concentration by the equation:
     \[
     \text{pOH} = -\log [\text{OH}^-]
     \]
   - Rearranging to solve for \([\text{OH}^-]\):
     \[
     [\text{OH}^-] = 10^{-\text{pOH}}
     \]

### Example Calculation:

Let's say the pH of the solution is 5.

1. **Calculate pOH**:
   \[
   \text{pOH} = 14 - 5 = 9
   \]

2. **Calculate \([\text{OH}^-]\)**:
   \[
   [\text{OH}^-] = 10^{-9} \, \text{mol/L}
   \]

### Final Answer:
The hydroxyl ion concentration \([\text{OH}^-]\) in a solution with a pH of 5 would be \(1 \times 10^{-9}\) mol/L.

Would you like more details on this process or have additional questions?

Here are some related questions:
1. What happens to \([\text{OH}^-]\) if the pH increases?
2. How does the \([\text{H}^+]\) concentration relate to pH?
3. How would you calculate the pH of a solution if \([\text{OH}^-]\) is known?
4. What is the relationship between pH and the strength of an acid or base?
5. How does dilution affect the pH and \([\text{OH}^-]\) of a solution?

**Tip**: Remember that pH and pOH are logarithmic scales. A change of 1 unit in pH corresponds to a tenfold change in \([\text{H}^+]\) or \([\text{OH}^-]\).

Learn how to calculate the hydroxyl ion concentration [OH-] in a solution based on its pH value. This guide provides step-by-step instructions, including formulas pH + pOH = 14 and pOH = -log[OH-]. Example calculation included.

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