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) \(\leq 7\) (d) \(14.2\)

Step-by-Step Solution

Answer

The pH of a solution containing \( \mathrm{MgCl}_2 \) is \( \leq 7 \) (option c).

Step 1: Identify Compound Dissociation

When magnesium chloride, \( \mathrm{MgCl}_2 \), dissolves in water, it dissociates into magnesium ions, \( \mathrm{Mg}^{2+} \), and chloride ions, \( \mathrm{Cl}^- \). The dissociation in water is: \[ \mathrm{MgCl}_2 \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{Cl}^-(aq) \]

Step 2: Consider the Impact of Ions on pH

We need to determine how these ions affect the pH of the solution. \( \mathrm{Cl}^- \) is the conjugate base of a strong acid, \( \mathrm{HCl} \), which means it does not affect pH significantly. \( \mathrm{Mg}^{2+} \), as a small, highly charged metal cation, can hydrolyze in water to form a slightly acidic solution.

Step 3: Predict the pH Based on Ion Effects

Since \( \mathrm{Cl}^- \) does not affect the pH and \( \mathrm{Mg}^{2+} \) can lead to the formation of \( \mathrm{H}^+ \) ions in water, the solution is slightly acidic. Hence, the pH of the \( \mathrm{MgCl}_2 \) solution should be \( \leq 7 \).

Key Concepts

MgCl2 dissociation

When magnesium chloride, \( \mathrm{MgCl}_2 \), dissolves in water, it dissociates into ions. This process is known as dissociation. Initially, \( \mathrm{MgCl}_2 \) is a solid compound formed by magnesium and chloride ions. When placed in water, it breaks down into its constituent ions: \( \mathrm{Mg}^{2+} \) and \( \mathrm{Cl}^- \). The equation for this process is: \[ \mathrm{MgCl}_2 \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{Cl}^-(aq) \] For dissociation:

• The solid \( \mathrm{MgCl}_2 \) transforms into electrolytic ions.
• Each magnesium ion is surrounded by water molecules, becoming the \( \mathrm{Mg}^{2+}(aq) \) complex.
• Similarly, each chloride ion becomes \( \mathrm{Cl}^- (aq) \) as water molecules surround it.

Dissociation plays an essential role in determining the subsequent properties of the solution, particularly its pH.

effect of ions on pH

The pH of a solution indicates its acidity or basicity. When \( \mathrm{MgCl}_2 \) dissociates in water, it forms \( \mathrm{Mg}^{2+} \) and \( \mathrm{Cl}^- \) ions. To understand the pH, it's crucial to determine how these ions interact in solutions:

• \( \mathrm{Cl}^- \): This ion is the conjugate base of the strong acid \( \mathrm{HCl} \). Since strong acids completely dissociate, \( \mathrm{Cl}^- \) ions do not alter the pH of the solution.
• \( \mathrm{Mg}^{2+} \): These cations can hydrolyze, meaning they react with water to produce \( \mathrm{H}^+ \) ions. This reaction creates a slightly acidic environment.

The presence of \( \mathrm{Mg}^{2+} \) ions slightly decreases the pH. Therefore, even though \( \mathrm{Cl}^- \) ions are neutral, the solution overall becomes somewhat acidic due to the \( \mathrm{Mg}^{2+} \) ions' impact.

hydrolysis of metal cations

Hydrolysis is a chemical process where molecules react with water, often leading to a change in pH. Metal cations like \( \mathrm{Mg}^{2+} \) can undergo hydrolysis, especially if they are small and highly charged. Through hydrolysis, \( \mathrm{Mg}^{2+} \) can form weak acids in the solution:

• The \( \mathrm{Mg}^{2+} \) interacts with water, leading to the formation of \( \mathrm{H}^+ \) ions (protons).
• These protons result in an acidic environment, since the increase in \( \mathrm{H}^+ \) concentration lowers the pH.
• Higher concentrations of \( \mathrm{Mg}^{2+} \) increase this acidic effect due to more extensive hydrolysis.

Hydrolysis of \( \mathrm{Mg}^{2+} \) ions does not lead to strong acidity but slightly shifts the pH below 7, indicating a mild acidic character in the \( \mathrm{MgCl}_2 \) solution.