which of these correctly defines the poh of a solution?

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29-03-2025

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Understanding pH is crucial in chemistry, but its counterpart, pOH, is often a source of confusion. This article clarifies the definition of pOH and helps you differentiate it from pH. We'll explore the correct definition and how it relates to the concentration of hydroxide ions in a solution.

What is pOH?

The pOH of a solution is a measure of the hydroxide ion (OH⁻) concentration. Unlike pH, which focuses on hydrogen ions (H⁺), pOH directly reflects the basicity or alkalinity of a solution. A lower pOH indicates a more basic solution, while a higher pOH suggests a more acidic solution.

The Correct Definition

The correct definition of pOH is the negative logarithm (base 10) of the hydroxide ion concentration:

pOH = -log₁₀[OH⁻]

Where [OH⁻] represents the molar concentration of hydroxide ions.

This formula is analogous to the pH formula: pH = -log₁₀[H⁺]. The key difference lies in the ion being measured: hydroxide ions for pOH and hydrogen ions for pH.

Understanding the Relationship Between pH and pOH

pH and pOH are inversely related in aqueous solutions at 25°C (room temperature). Their relationship is governed by the following equation:

pH + pOH = 14

This means that if you know the pH of a solution, you can easily calculate its pOH, and vice-versa. For example, a solution with a pH of 3 has a pOH of 11 (14 - 3 = 11).

Common Misconceptions about pOH

One common misconception is that pOH directly measures acidity. While a higher pOH correlates with a lower concentration of hydroxide ions and therefore a more acidic solution, it's the pH that's the primary indicator of acidity. P0H indicates alkalinity.

Another misconception is that pOH is only relevant for strongly basic solutions. In reality, pOH applies to all aqueous solutions, regardless of their acidity or basicity. It provides an alternative perspective on the solution's hydroxide ion concentration.

How to Calculate pOH

Calculating pOH is straightforward once you know the hydroxide ion concentration.

Example: Let's say a solution has a hydroxide ion concentration of 1 x 10⁻⁵ M.

1. Substitute the concentration into the pOH formula: pOH = -log₁₀(1 x 10⁻⁵)
2. Calculate the logarithm: The log₁₀ of 1 x 10⁻⁵ is -5.
3. Apply the negative sign: pOH = -(-5) = 5

Therefore, the pOH of this solution is 5.

Importance of pOH

While pH is more commonly used, pOH offers a valuable perspective on the alkalinity of a solution. In fields like environmental science and chemical engineering, monitoring both pH and pOH can provide a more comprehensive understanding of the solution's properties.

Conclusion

The correct definition of pOH is the negative logarithm (base 10) of the hydroxide ion concentration. Understanding the relationship between pH and pOH, and how to calculate pOH, is essential for anyone working with aqueous solutions in chemistry. Remember, a lower pOH indicates a more basic solution, and pOH, alongside pH, provides a complete picture of a solution's chemical characteristics.