Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

A base dissolved in water yields a solution with a hydroxyl ion concentration of \(0.05\) mol litre \(^{-1}\). The solution is (a) basic (b) acid (c) neutral (d) either (b) or (c)

Step-by-Step Solution

Answer

(a) basic

Step 1: Understanding Neutral pH

To determine whether a solution is acidic, basic, or neutral, note that a neutral solution has a hydroxyl ion concentration \\([OH^-]\) of equal to \(1 \times 10^{-7}\) mol/L at 25°C because in pure water, [H+] and [OH-] are both \(1 \times 10^{-7}\) mol/L.

Step 2: Compare Hydroxyl Ion Concentrations

The given hydroxyl ion concentration of the solution is \(0.05\) mol/L. Compare this with the neutral concentration of \(1 \times 10^{-7}\) mol/L. The value \(0.05 > 1 \times 10^{-7}\) suggests that the solution has a higher concentration of \(OH^-\) ions than a neutral solution.

Step 3: Determine the Nature of the Solution

A solution with a hydroxyl ion concentration greater than \(1 \times 10^{-7}\) mol/L indicates that the solution is basic because there are more \(OH^-\) ions compared to a neutral state.

Key Concepts

Hydroxyl Ion Concentration

When we discuss hydroxyl ion concentration, we are essentially talking about the amount of hydroxyl ions, denoted as \(OH^-\), in a solution. This is an important factor in chemistry as it helps determine the acidity or basicity of a solution. In a solution, the concentration is usually measured in moles per liter (mol/L).

In pure water at 25°C, a neutral environment, the concentration of both hydrogen ions \([H^+]\) and hydroxyl ions \([OH^-]\) is \(1 \times 10^{-7}\) mol/L. This balance is what keeps the solution neutral. When the hydroxyl ion concentration deviates from this balance, it influences the solution's pH, making it more acidic or basic.

If the concentration of \(OH^-\) is greater than \(1 \times 10^{-7}\) mol/L, like in our given exercise at \(0.05\) mol/L, there's an overabundance of hydroxyl ions, indicating the solution leans towards being basic.

Basic Solution Determination

Determining whether a solution is basic involves understanding the relationship between hydroxyl ions and the pH scale. The pH scale, which ranges from 0 to 14, measures how acidic or basic a solution is—with lower numbers being acidic, 7 being neutral, and higher numbers being basic.

The presence of more hydroxyl ions \([OH^-]\) than hydrogen ions \([H^+]\) in a solution results in a basic solution. For the exercise in question, the given concentration of hydroxyl ions is \(0.05\) mol/L, which is significantly higher than the neutral concentration of \(1 \times 10^{-7}\) mol/L.

• This surplus of \(OH^-\) ions shifts the balance towards a basic pH.
• In such a condition, we can safely determine the solution is basic without even calculating the exact pH, as the concentration difference is quite clear.

Recognizing the presence and dominance of hydroxyl ions allows for straightforward determination of a solution's basic nature.

Neutral pH Comparison

Neutral pH is a pivotal point on the pH scale, sitting at precisely 7. In essence, a neutral pH indicates a perfect balance between hydrogen ions \([H^+]\) and hydroxyl ions \([OH^-]\). This is a characteristic of pure water at 25°C where both ion concentrations are \(1 \times 10^{-7}\) mol/L.

Comparing a solution's hydroxyl ion concentration with that of a neutral solution gives insight into the solution's nature. If the concentration of \([OH^-]\) exceeds \(1 \times 10^{-7}\) mol/L, the solution shifts away from neutrality into basic territory, as seen in the exercise where \(0.05\) mol/L concentration implies a basic solution.

• If \(OH^-\) concentrations were less than \(1 \times 10^{-7}\) mol/L, the solution might become acidic.
• Understanding this threshold is crucial for interpreting chemical compositions and their effects.

Hence, assessing neutrality requires careful comparison of ion concentrations relative to the neutral baseline of water.