Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

Solution of \(\mathrm{MgCl}_{2}\) in water has \(\mathrm{pH}\) value (a) 7 (b) \(>7\) (c) \(<7\) (d) \(14.2\)

Step-by-Step Solution

Answer

The pH of \(\text{MgCl}_2\) solution is less than 7, so option (c) is correct.

Step 1: Identify the Nature of \(\text{MgCl}_2\)

\(\text{MgCl}_2\) is a salt derived from a strong acid (HCl) and a weak base (Mg(OH)\(_2\)). This indicates that its solution in water will be acidic.

Step 2: Determine the Ionization in Water

When \(\text{MgCl}_2\) dissolves in water, it dissociates into \(\text{Mg}^{2+}\) and \(2 \text{Cl}^-\). However, the \(\text{Mg}^{2+}\) ion can interact with water molecules, slightly increasing acidity.

Step 3: Evaluate the pH Based on Ions Present

\(\text{Mg}^{2+}\) from \(\text{MgCl}_2\) can hydrolyze partially, forming \(\text{Mg(OH)}^+\) and \(\text{H}^+\), thus making the solution acidic. This means the pH is likely to be less than 7.

Step 4: Conclusion on pH Value

Given the acidic nature of \(\text{MgCl}_2\) in solution, the pH value will be less than 7.

Key Concepts

Acidic Solutions

When we talk about acidic solutions, we refer to solutions that have a pH less than 7. The pH scale measures how acidic or basic a solution is. For solutions that are acidic, there is a higher concentration of hydrogen ions (\( ext{H}^+\)) than hydroxide ions (\( ext{OH}^-\)).

It's essential to understand that pH is a logarithmic scale. This means that each whole number change represents a tenfold change in acidity. So, a solution with a pH of 6 is ten times more acidic than one with a pH of 7. Understanding this scale is crucial in predicting the properties of various solutions, such as their corrosive behavior or reactivity. For example:

• Vinegar and lemon juice are common examples of acidic solutions.
• Acidic solutions typically taste sour and can conduct electricity due to the presence of free ions.
• They are often involved in chemical reactions, especially those seeking to neutralize basic compounds.

The acidic nature of a solution, like that of \( ext{MgCl}_2\), results from the dissociation of salts derived from strong acids and weak bases.

Hydrolysis of Salts

Hydrolysis of salts refers to a reaction where a salt reacts with water to either produce an acidic or basic solution. This occurs because of the ions present in the salt, which can interact with water molecules. When a salt like \( ext{MgCl}_2\) is dissolved in water, it dissociates into its constituent ions, \( ext{Mg}^{2+}\) and \( ext{Cl}^-\).

The key ion to focus on in this process is \( ext{Mg}^{2+}\). This ion can interact with water molecules to release hydrogen ions (\( ext{H}^+\)), leading to increased acidity in the solution:

• The \( ext{Mg}^{2+}\) ion partially hydrolyzes with water, forming \( ext{Mg(OH)}^+\) and releasing \( ext{H}^+\).
• This release of \( ext{H}^+\) ions decreases the pH of the solution, confirming its acidic nature.
• Hydrolysis reactions like this are common for salts formed from strong acids and weak bases.

Understanding hydrolysis helps explain why solutions derived from certain salts are not neutral. It draws a connection between the types of ions present and the resultant pH of the solution.

Strong Acids and Weak Bases

A critical concept in understanding the pH of salt solutions is the origin of the salt itself, particularly from strong acids and weak bases. The pH character of a salt solution is heavily influenced by the strength of the acid and base it originates from.

• A strong acid, like HCl, dissociates completely in water, releasing a large number of \( ext{H}^+\) ions.
• Weak bases like \( ext{Mg(OH)}_2\) do not fully dissociate, producing fewer \( ext{OH}^-\) ions in solution.

When a salt like \( ext{MgCl}_2\) is formed from these components, it tends to make the resultant solution acidic:

• The strong acid part contributes numerous hydrogen ions, overpowering the basic part of the solution.
• This imbalance is why \( ext{MgCl}_2\) solutions are acidic.

This understanding underscores the importance of the source substances in predicting the behavior of a solution. By knowing whether a salt comes from a strong acid and a weak base, you can predict whether it will result in an acidic solution.