Problem 36 - Chemistry Matter and Change

Question

Describe how you can determine the pH of a solution if you know its poH.

Step-by-Step Solution

Answer

To determine the pH of a solution if you know its poH, you can use the relationship between pH and poH, which is pH + poH = 14 for aqueous solutions at 25°C. Rearrange the equation to find the pH value: pH = 14 - poH. Substitute the given poH value and solve for the pH value. The obtained pH value indicates the acidity or alkalinity of the solution, with values less than 7 being acidic, greater than 7 being alkaline, and equal to 7 being neutral.

Step 1: Understand the relationship between pH, poH, [H+], and [OH-]

Before we determine the pH of the solution given its poH, it is essential to understand the relationship between pH, poH, [H+] (concentration of hydrogen ions), and [OH-] (concentration of hydroxide ions). The pH and poH are defined by the following formulas: pH = -log10([H+]) poH = -log10([OH-]) Additionally, for any aqueous solution at 25°C, the following relationship between pH and poH holds true: pH + poH = 14

Step 2: Substitute the given poH value into the equation

Now that we are aware of the relationship between pH and poH, we will substitute the given poH value into the equation to find the pH of the solution. To do so, we can rewrite the equation: pH = 14 - poH Substitute the given poH value and solve for the pH value.

Step 3: Calculate and interpret the pH value

After substituting the poH value and solving for the pH value, you will obtain the pH of the solution. The pH value is a measure that indicates the acidity or alkalinity of a solution. A pH value less than 7 indicates that the solution is acidic, while a pH value greater than 7 corresponds to an alkaline solution. If the pH is equal to 7, the solution is neutral. By determining the pH of the solution, you can now analyze and understand the properties of the given solution, such as its acidity or alkalinity, and predict how it might react with other substances.

Key Concepts

pH and poH relationship

Understanding the connection between pH and poH is crucial when dealing with solutions. These two terms describe the concentrations of hydrogen ions \([H^+]\) and hydroxide ions \([OH^-]\) in a solution. To calculate pH, we use the formula \( pH = -\log_{10}([H^+]) \), and for poH, \( poH = -\log_{10}([OH^-]) \). Together, these formulas help us determine the nature of the solution—whether it's acidic, neutral, or basic.

In any aqueous solution at 25°C, pH and poH are linked by the equation:

• \( pH + poH = 14 \)

This equations means that if you know one value, you can easily calculate the other. So, if you’re given a poH value, subtract it from 14 to find the pH. This relationship simplifies understanding how a solution behaves based on its hydrogen and hydroxide ion concentrations.

Acidity and Alkalinity

The concepts of acidity and alkalinity describe the chemical nature of solutions. These concepts rely heavily on the pH scale, which ranges from 0 to 14. Each number on the scale correlates to the concentration of hydrogen ions in the solution.

When you have a solution:

• A pH less than 7 indicates an acidic solution, which has more hydrogen ions \([H^+]\).
• A pH of 7 means a neutral solution, like pure water.
• A pH greater than 7 means a basic (or alkaline) solution, with fewer hydrogen ions and more hydroxide ions \([OH^-]\).

This understanding is essential because the pH can affect everything from chemical reactions to biological processes. In practical terms, this means knowing the pH can help you predict how a solution will interact with different substances, its effects on the environment, or its suitability for various industrial processes.

Chemical Reactions in Solutions

Chemical reactions in solutions often depend on the concentration of hydrogen ions and the pH level. Acids and bases react in predictable ways when dissolved in water, undergoing reactions that can either form or break down substances.

For example, when an acid reacts with a base, the products are typically water and a salt, a process known as neutralization. The pH of the resulting solution helps to determine the completeness of the reaction:

• If the pH is 7, the reaction is perfectly balanced, indicating a complete neutralization.
• If the pH is lower than 7, the solution remains acidic after the reaction.
• If it is higher than 7, the solution is basic.

Understanding these reactions helps us predict and control the outcome of mixing different chemical solutions. Whether in a scientific lab or industrial setting, knowing how reactions affect pH is crucial for achieving desired results.