How To Find Oh From Ph

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How to Find OH- from pH

Introduction

In acid-base chemistry, pH is a crucial parameter that indicates the acidity or alkalinity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration ([H+]). However, in certain situations, it is necessary to determine the concentration of the hydroxide ion (OH-), which is inversely related to pH. This article provides a comprehensive guide on how to find OH- from pH, covering the underlying theory, formulas, and practical examples.

Relationship between pH and OH-

The relationship between pH and OH- is governed by the following equation:

pH + pOH = 14

where pOH is the negative logarithm of OH- concentration. This equation implies that in aqueous solutions at 25 °C:

  • If pH is low (acidic), OH- concentration is high (basic).
  • If pH is high (basic), OH- concentration is low (acidic).

Formula for Calculating OH- from pH

Rearranging the relationship equation, we get the formula for calculating OH- concentration from pH:

[OH-] = 10^-pOH = 10^(14 – pH)

Practical Example

Let’s say we have a solution with a pH of 9. Calculate the OH- concentration:

pOH = 14 – pH = 14 – 9 = 5

[OH-] = 10^-pOH = 10^-5 = 1 x 10^-5 M

Tips for Accurate Results

When calculating OH- from pH, keep the following tips in mind:

  • Use accurate pH values.
  • Ensure that the temperature of the solution is 25 °C.
  • Consider the limitations of the pH meter or indicator used.

Conclusion

Finding OH- concentration from pH is a straightforward process using the formula [OH-] = 10^(14 – pH). Understanding this relationship is essential in various applications, including acid-base titrations, buffer preparation, and water quality analysis. By following the steps and tips outlined in this guide, you can accurately calculate OH- concentration and gain a better understanding of acid-base chemistry.

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