Calculate The Ph of A 0.42 M Magnesium Hydroxide Solution.

Magnesium hydroxide (Mg(OH)₂) is a strong base that dissociates completely in water. Calculating its pH involves determining the concentration of hydroxide ions ([OH⁻]) and then using the pH formula. This guide explains the process step-by-step with an interactive calculator.

Introduction

The pH of a solution is a measure of its acidity or basicity, with values below 7 indicating acidity and above 7 indicating basicity. For magnesium hydroxide solutions, the pH can be calculated using the concentration of hydroxide ions.

Magnesium hydroxide is a strong electrolyte, meaning it dissociates completely in water according to the equation:

Dissociation Equation

Mg(OH)₂ (s) → Mg²⁺ (aq) + 2OH⁻ (aq)

For a 0.42 M solution of magnesium hydroxide, the concentration of hydroxide ions is twice the concentration of magnesium hydroxide because each formula unit produces two hydroxide ions.

pH Calculation Formula

The pH of a solution is calculated using the concentration of hydroxide ions with the following formula:

pH Formula

pH = -log₁₀[OH⁻]

For magnesium hydroxide solutions, the concentration of hydroxide ions is equal to twice the concentration of magnesium hydroxide:

Concentration Relationship

[OH⁻] = 2 × [Mg(OH)₂]

Combining these, the pH of a magnesium hydroxide solution can be calculated as:

Complete Formula

pH = -log₁₀(2 × [Mg(OH)₂])

Worked Example

Let's calculate the pH of a 0.42 M magnesium hydroxide solution step-by-step.

Determine the concentration of hydroxide ions: [OH⁻] = 2 × 0.42 M = 0.84 M
Calculate the pH using the pH formula: pH = -log₁₀(0.84)
Compute the logarithm: log₁₀(0.84) ≈ 0.9242
Take the negative: pH ≈ -0.9242 ≈ 11.0758

Result

The pH of a 0.42 M magnesium hydroxide solution is approximately 11.08.

Interpreting Results

A pH of 11.08 indicates a strongly basic solution. This is expected for magnesium hydroxide, which is a strong base. The solution will feel slippery and will turn red litmus paper blue.

For comparison, here's a table showing pH ranges and their characteristics:

pH Range	Characteristics
0-6	Acidic (e.g., lemon juice, vinegar)
7	Neutral (e.g., pure water)
8-14	Basic (e.g., baking soda, soap)

FAQ

Why does magnesium hydroxide have a high pH?: Magnesium hydroxide is a strong base that completely dissociates in water, releasing two hydroxide ions per formula unit, resulting in a high concentration of OH⁻ ions and thus a high pH.
Can I use this calculator for other bases?: This calculator is specifically designed for magnesium hydroxide solutions. For other bases, you would need to adjust the dissociation factor accordingly.
What if my solution is diluted?: The calculator assumes the solution is undiluted. For diluted solutions, you would need to account for the dilution factor in the concentration calculation.
Is magnesium hydroxide safe to handle?: Magnesium hydroxide is generally safe when handled properly, but it can cause skin and eye irritation. Always wear appropriate protective equipment when working with chemical solutions.