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

The solution is (a) basic.

Step 1: Understanding pH and pOH

To determine if the solution is basic, acidic, or neutral, we need to understand the relationship between pH, pOH, and ion concentration in a solution. A solution is acidic if its pH is less than 7, basic if more than 7, and neutral if exactly 7 at 25°C. pOH is similarly defined, but deals with hydroxyl ions.

Step 2: Calculating the pOH of the Solution

The concentration of hydroxyl ions \([ ext{OH}^-]\) is given as 0.05 mol/L. The formula for pOH is: \[ \text{pOH} = -\log[\text{OH}^-] \]Substituting the given concentration into this formula gives: \[ \text{pOH} = -\log(0.05) \approx 1.3 \]

Step 3: Relating pOH to pH

We know from the relationship between pH and pOH that: \[ \text{pH} + \text{pOH} = 14 \]Plugging the calculated pOH into this equation: \[ \text{pH} = 14 - 1.3 = 12.7 \]

Step 4: Classifying the Solution

Since the calculated pH is 12.7, which is greater than 7, the solution is classified as basic. A pH greater than 7 at 25°C indicates that the solution is a base.

Key Concepts

pH and pOH relationship

The concepts of pH and pOH are fundamental to understanding the acidity or basicity of a solution. These two measures are closely interconnected, capturing the balance between hydrogen ions \([\text{H}^+]\) and hydroxide ions \([\text{OH}^-]\) in a solution.
At a standard temperature of 25°C, the sum of pH and pOH is always 14. This results from the self-ionization of water, where the concentration of \([\text{H}^+]\) and \([\text{OH}^-]\) gives a neutral pH of 7. If a solution has a pH less than 7, it is acidic, meaning it has a higher concentration of hydrogen ions. Conversely, a pH greater than 7 indicates a basic solution, with a higher concentration of hydroxide ions.
The pOH is calculated by the formula: \[ \text{pOH} = -\log[\text{OH}^-] \]This measures the concentration of hydroxide ions, complementing the pH, which measures hydrogen ion concentration. Together, they enable us to classify solutions effectively.

ionic concentration in solutions

The concentration of ions in a solution is pivotal in determining its pH or pOH.
For instance, when a base dissolves in water, it increases the concentration of hydroxide ions \([\text{OH}^-]\).
This ion concentration directly affects the pOH, which in turn influences the pH.
Using the formula for pOH, \[ \text{pOH} = -\log[\text{OH}^-] \],you can determine the degree of basicity of the solution.
A higher hydroxide ion concentration leads to a lower pOH and consequently a higher pH, indicating a more basic solution.
The understanding of ion concentration is essential for effectively predicting chemical behavior in various reactions.

acid-base classification

In chemistry, acid-base classification is central to predicting the behavior of solutions.
Solutions are typically categorized as acidic, basic, or neutral based on their pH values.
An acidic solution, with a pH below 7, contains more hydrogen ions \([\text{H}^+]\) than hydroxide ions. Acids are known for their sour taste and ability to turn blue litmus paper red.
Basic solutions, or alkaline solutions, have a pH above 7 and contain more hydroxide ions \([\text{OH}^-]\), which can feel slippery and turn red litmus paper blue.
Neutral solutions have a balanced concentration of hydrogen and hydroxide ions, typically resulting in a pH of 7.
This classification is crucial for understanding how different substances will interact in a solution, aiding in everything from formulating household cleaners to pharmaceuticals.