Question

Compare the results of two experiments: First, a small amount of base is added to an unbuffered solution with a pH of 7. Second, the same amount of base is added to a buffered solution with a pH of 7.

Short answer

In summary, adding a small amount of base to an unbuffered solution with a pH of 7 will result in a significant increase in pH (a more alkaline solution) due to the absence of a buffering system. On the other hand, adding the same amount of base to a buffered solution with a pH of 7 will have a much smaller effect on pH, as the buffer (weak acid and its conjugate base) helps maintain the pH by reacting with the added base. The key difference between the two experiments is the presence of a buffering system in the buffered solution, which efficiently resists the pH changes upon addition of the base.

Step-by-step solution

Understand pH and buffering

pH is a measure of the acidity or alkalinity of a solution, given by the formula: \[pH = -\log_{10} [H^+]\] Where \( [H^+] \) is the concentration of hydrogen ions in the solution. Buffered solutions are those that can resist changes in pH upon the addition of small amounts of acid or base. This is usually due to the presence of a weak acid and its conjugate base, or vice versa.

Adding a base to the unbuffered solution

Since the initial pH of the unbuffered solution is 7, it means that it is neutral i.e., the concentration of hydrogen ions ([H+]) equals the concentration of hydroxide ions ([OH-]). When a small amount of base is added to this solution, it increases the concentration of hydroxide ions. According to the equation: \[14 = pH + pOH\] The increase in hydroxide ion concentration (or decrease in pOH) will lead to a decrease in hydrogen ion concentration, thus increasing the pH of the solution.

Adding a base to the buffered solution

When the same amount of base is added to the buffered solution, the presence of a buffer will counteract the change in pH. This is due to the weak acid and its conjugate base components, which can react with the added base, maintaining a nearly constant pH despite the addition of more hydroxide ions. The change in pH will be much smaller than in the unbuffered solution.

Comparing the two experiments

In conclusion: 1. Adding a small amount of base to the unbuffered solution with a pH of 7 will result in an increased pH (a more alkaline solution). The change in pH will be significant since there is no buffering system in place. 2. Adding the same amount of base to the buffered solution with a pH of 7 will have a much smaller effect on the pH compared to the unbuffered solution. This is because of the presence of the buffering system (weak acid and its conjugate base) in the buffered solution that helps maintain the pH. The main difference between the two experiments lies in the effectiveness of the buffering system in mitigating the pH change when a base is added to the solution.

Key concepts

pH and Buffering

The concept of pH is crucial for understanding the acidity or alkalinity of a solution. It is a logarithmic scale based on the concentration of hydrogen ions \( [H^+] \) in the solution. The formula \( pH = -\log_{10} [H^+] \) allows us to quantify the pH level, where lower pH values represent acidic solutions and higher pH values indicate alkaline solutions.

Buffering comes into play when we need to maintain the pH of a solution. A buffer is a system that minimizes changes in pH levels. When acids or bases are added to buffered solutions, the pH change is much less than it would be in unbuffered solutions. This is because buffers are typically made up of a weak acid and its conjugate base or vice versa, which can absorb excess \( H^+ \) or \( OH^- \) ions without significantly altering the pH.

pH Scale and Calculations

Understanding the pH scale is essential for performing calculations that involve acidity or alkalinity. The pH scale ranges from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are acidic, and those with a pH greater than 7 are basic or alkaline.

To calculate the pH, we often use the relationship between pH and \( pOH \) with the equation \(14 = pH + pOH\). If you know the concentration of hydrogen ions or hydroxide ions, you can determine one value and then easily calculate the other, thus finding the pH.

Acid-Base Neutralization

Acid-base neutralization is the process in which an acid and a base react to form water and a salt. This is an example of a double displacement reaction.

In an unbuffered solution, the addition of a base leads to an increase in the hydroxide ion concentration, moving the pH to a more alkaline value. The reaction between the acid (with \( H^+ \) ions) and the base (with \( OH^- \) ions) should ideally result in a pH close to 7, which is neutral; however, the exact pH after neutralization depends on the strength of the acid and base involved.

Weak Acids and Conjugate Bases

A weak acid is one that does not fully dissociate in water, meaning it does not release all its \( H^+ \) ions into the solution. Conversely, a strong acid fully dissociates and releases all its \( H^+ \) ions.

A conjugate base is formed when a weak acid loses a proton \( H^+ \) ion. Together, the weak acid and its conjugate base form a buffer pair, which can dynamically adjust concentrations to maintain the overall pH of the solution. This concept is key in buffering as it allows for the 'absorption' of the new \( OH^- \) or \( H^+ \) without significantly affecting the pH.