Calculate The Poh of The Following Solutions at 25c

Calculating the pOH of a solution at 25°C is essential in chemistry for understanding acidity and alkalinity. This guide provides a step-by-step method to determine pOH values, along with practical examples and interpretation tips.

Introduction

The pOH scale measures the concentration of hydroxide ions (OH⁻) in a solution. At 25°C, the relationship between pOH and the concentration of hydroxide ions is defined by the formula:

pOH = -log[OH⁻]

Where [OH⁻] is the molar concentration of hydroxide ions in moles per liter (mol/L). The pOH scale ranges from 0 to 14, with lower values indicating more alkaline solutions.

pOH Formula

The pOH of a solution can be calculated using the following formula:

pOH = -log₁₀[OH⁻]

Where:

[OH⁻] = concentration of hydroxide ions in mol/L
log₁₀ = logarithm base 10

This formula is derived from the definition of pOH as the negative logarithm of the hydroxide ion concentration.

How to Calculate pOH

To calculate the pOH of a solution at 25°C, follow these steps:

Determine the concentration of hydroxide ions [OH⁻] in the solution in mol/L.
Take the negative logarithm (base 10) of the hydroxide ion concentration.
The result is the pOH value of the solution.

Note: The calculation assumes the solution is at 25°C. For other temperatures, the relationship between pOH and [OH⁻] changes.

Worked Examples

Let's calculate the pOH for two different solutions:

Example 1: Strong Base Solution

Consider a 0.10 mol/L NaOH solution at 25°C.

Given: [OH⁻] = 0.10 mol/L

Calculation: pOH = -log(0.10) = 1.00

Result: The pOH of the solution is 1.00.

Example 2: Weak Base Solution

Consider a 0.0010 mol/L NH₃ solution at 25°C.

Given: [OH⁻] = 0.0010 mol/L

Calculation: pOH = -log(0.0010) = 3.00

Result: The pOH of the solution is 3.00.

Interpreting pOH Values

The pOH value provides information about the alkalinity of a solution:

pOH < 7: The solution is alkaline (basic).
pOH = 7: The solution is neutral (like pure water).
pOH > 7: The solution is acidic.

For example, a pOH of 10 indicates a very alkaline solution with a high concentration of hydroxide ions.

FAQ

What is the relationship between pOH and pH?: The pOH and pH of a solution are related by the equation: pH + pOH = 14 at 25°C. This is because water ionizes to form equal concentrations of H⁺ and OH⁻ ions in neutral solutions.
Can pOH be calculated for solutions at temperatures other than 25°C?: Yes, but the relationship between pOH and [OH⁻] changes with temperature. The formula pOH = -log[OH⁻] remains valid, but the ionization constant of water (Kw) must be adjusted for the specific temperature.
What is the pOH of pure water at 25°C?: The pOH of pure water at 25°C is 7.00, as the concentration of hydroxide ions is 1.0 × 10⁻⁷ mol/L.
How does pOH relate to the strength of a base?: A stronger base will dissociate more completely in water, producing a higher concentration of hydroxide ions and a lower pOH value. For example, NaOH (a strong base) has a lower pOH than NH₃ (a weak base) at the same concentration.