Question

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

Short answer

(c) \(<7\)

Step-by-step solution

Understand the Composition of MgCl2

Magnesium chloride, represented as \( \mathrm{MgCl}_{2} \), is an ionic compound made up of \( \mathrm{Mg}^{2+} \) ions and two \( \mathrm{Cl}^{-} \) ions. It completely dissociates in water to form these ions.

Analyze the Effect of \( \mathrm{Mg}^{2+} \)

The \( \mathrm{Mg}^{2+} \) ion is considered a weak acid because it can hydrolyze slightly in water, forming \( \mathrm{Mg(OH)}^{+} \) and releasing \( \mathrm{H}^{+} \) ions, which could decrease the \( \mathrm{pH} \) of the solution a bit. However, this effect is minimal.

Analyze the Effect of \( \mathrm{Cl}^{-} \)

The \( \mathrm{Cl}^{-} \) ion does not react with water and neither increases nor decreases the \( \mathrm{pH} \) because it is the conjugate base of \( \mathrm{HCl} \), a strong acid.

Determine the Overall Effect on pH

Since \( \mathrm{Mg}^{2+} \) can slightly decrease the \( \mathrm{pH} \), but not significantly, the solution of \( \mathrm{MgCl}_2 \) in water will likely have a \( \mathrm{pH} \) that is close to or just below 7.

Key concepts

Magnesium Chloride

Magnesium chloride, denoted as \( \mathrm{MgCl}_{2} \), is a chemical compound often used in various industrial and chemical applications. At its core, \( \mathrm{MgCl}_{2} \) is a salt composed of magnesium and chlorine ions. In a solid state, this compound contains one magnesium ion and two chloride ions held together by ionic bonds. When dissolved in water, \( \mathrm{MgCl}_{2} \) readily dissociates into its constituent ions, which each play a role in determining the solution's properties.
When magnesium chloride dissolves, it dissociates completely into \( \mathrm{Mg}^{2+} \) and \( \mathrm{Cl}^{-} \) ions.

Ionic Compounds

Ionic compounds like magnesium chloride consist of positively and negatively charged ions. These ions are formed when metals and non-metals chemically bond. The transfer of electrons from one element to another typically leads to the formation of ionic bonds. In \( \mathrm{MgCl}_{2} \), the magnesium ion \( \mathrm{Mg}^{2+} \) donates electrons to the chloride ions \( \mathrm{Cl}^{-} \), resulting in an ionic bond.

• Ions in an ionic compound are usually metals and non-metals.
• Ionic bonds result from the electrostatic forces between the oppositely charged ions.
• When dissolved in water, these ionic compounds generally break apart into their constituent ions.

Hydrolysis

Hydrolysis refers to a chemical process where a compound reacts with water, often leading to the breakdown of that compound. In the context of magnesium chloride, although \( \mathrm{Cl}^{-} \) is inert in water, \( \mathrm{Mg}^{2+} \) can undergo slight hydrolysis. This reaction with water results in the formation of \( \mathrm{Mg(OH)}^{+} \) and releases protons \( \mathrm{H}^{+} \) ions.
This interaction is why \( \mathrm{Mg}^{2+} \) is perceived as slightly acidic—it has the potential to decrease the pH slightly when dissolved in water. This hydrolysis effect is quite weak but still contributes to the overall pH level of a magnesium chloride solution potentially being less than 7.

Weak Acid Effect

The concept of a weak acid effect comes into play when we observe the way \( \mathrm{Mg}^{2+} \) interacts with water. Unlike strong acids that fully dissociate in water producing a significant amount of \( \mathrm{H}^{+} \) ions, \( \mathrm{Mg}^{2+} \) as a weak acid only partially contributes to the acidity of a solution.

• Only a minimal amount of protons are released due to the weak hydrolysis of \( \mathrm{Mg}^{2+} \).
• The presence of any added \( \mathrm{H}^{+} \) can slightly lower the solution's pH, but generally this effect is so small that the solution remains near neutral.
• Therefore, although \( \mathrm{Cl}^{-} \) does not affect the pH, \( \mathrm{Mg}^{2+} \) does introduce a weak acidic effect.

This means that a solution of \( \mathrm{MgCl}_{2} \) in water will have a pH that is close to neutral but slightly less than 7.