Question

Define pOH. Write an equation relating pH and pOH.

Short answer

pOH represents the negative logarithm of the concentration of hydroxide ions in a solution. The equation relating pH and pOH is \(pH + pOH = 14\) at 25°C.

Step-by-step solution

Define pOH

pOH is an important term used in chemistry that represents the negative logarithm (base 10) of the concentration of hydroxide ions (OH-) present in a solution. It provides an effective method to express the basic concentration.

Relate pH and pOH

pH and pOH have a direct relationship in that they will always add up to 14 at 25°C (standard room temperature). Therefore the equation which relates pH and pOH can be written as \(pH + pOH = 14\)

Note

The relationship between pH and pOH is established by the ion product of water, which is constant at a given temperature. The universal value of 14 is derived from the ion product of water (\(1.0 \times 10^{-14}\)) at 25°C. Should the temperature change, the pH + pOH = 14 rule would no longer apply as the ion product for water changes with temperature.

Key concepts

pH and pOH Relationship

Understanding the chemistry of acids and bases can often begin with exploring the interconnected concepts of pH and pOH. These two measures express the acidity and basicity of solutions respectively, and they are intrinsically linked.

The pH scale, which stands for 'potential of Hydrogen,' is a measure of the hydrogen ion concentration in a solution. In contrast, pOH represents the concentration of hydroxide ions. One of the most fundamental principles connecting these two is that at 25°C, the sum of pH and pOH in any aqueous solution is always 14.

This relationship is neatly summed up with the equation: \(pH + pOH = 14\). It's a simple yet powerful formula that allows chemists to calculate one value if the other is known, facilitating the understanding of a solution's overall chemical nature. For example, if a solution has a pH of 3, which is quite acidic, we can quickly deduce that the pOH would be 11, reflecting a very low concentration of hydroxide ions.

Hydroxide Ion Concentration

Diving deeper into the concept of pOH, it's crucial to grasp what hydroxide ion concentration is and why it matters. Hydroxide ions (\([OH^-]\)) are negatively charged and, along with hydrogen ions (\([H^+]\)), are key players in determining a solution's acidic or basic character.

The concentration of hydroxide ions in a solution is usually measured in moles per liter (M), and pOH is the negative base-10 logarithm of this concentration: \(pOH = -\log[OH^-]\). A higher concentration of hydroxide ions means a higher basicity or alkalinity of the solution, resulting in a lower pOH value. So, a low pOH corresponds to a high pH and vice versa.

By understanding pOH and its relationship to hydroxide ion concentration, students and chemists can manipulate and predict the behavior of chemical solutions, particularly in titrations and neutralization reactions where the goal may be to reach a certain pH level.

Ion Product of Water

At the core of the pH and pOH concepts is the ion product of water (Kw), which is vital for understanding aqueous chemistry. Water naturally dissociates into hydrogen and hydroxide ions, and at 25°C, the ion product for water (\([H^+][OH^-] = 1.0 \times 10^{-14}\)) is a constant value. This constant is what anchors the pH and pOH scale relationship.

Any change in the concentration of hydrogen ions will be mirrored by an opposite change in the concentration of hydroxide ions to maintain this constant value. Consequently, if pH shifts, so does pOH, but in a way that ensures the product of their respective ion concentrations stays locked at \(1.0 \times 10^{-14}\) at standard temperature.

However, one should note that Kw, and thus the relationship between pH and pOH, can shift with temperature. For those studying chemistry, this is a critical consideration when working in environments that differ from standard room temperature, as it means the 'magic number' 14 can change under these conditions, requiring temperature-specific adjustments for accurate calculations.