Determine the [OH^–], pH, and pOH of a 0.15 M ammonia solution.

Verified step by step guidance

1. 1. Ammonia is a weak base, so it will react with water to form ammonium ions and hydroxide ions. The reaction is: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq).
2. 2. To find the concentration of OH-, we need to use the Kb expression for ammonia. Kb is the base ionization constant, which is a measure of how much a base ionizes in solution. The Kb expression for ammonia is: Kb = [NH4+][OH-] / [NH3]. You can look up the Kb value for ammonia in a table, it is typically around 1.8 x 10^-5.
3. 3. Since we are dealing with a weak base, we can make the assumption that the change in concentration of NH3 is approximately equal to the concentration of OH- produced. Therefore, we can set up the Kb expression as follows: Kb = x^2 / (0.15 - x), where x is the concentration of OH-.
4. 4. Solve the above equation for x, which will give you the concentration of OH-. This involves some algebra and possibly the use of the quadratic formula. Remember that x is approximately equal to the concentration of OH-, so the concentration of OH- is approximately equal to the square root of (Kb * initial concentration of NH3).
5. 5. Once you have the concentration of OH-, you can find the pOH by taking the negative logarithm (base 10) of the OH- concentration. The pH can then be found by subtracting the pOH from 14, since pH + pOH = 14 at 25 degrees Celsius.

Key Concepts

Ammonia as a Weak Base

Ammonia (NH3) is a weak base that partially ionizes in water to form hydroxide ions (OH-) and ammonium ions (NH4+). The equilibrium reaction can be represented as NH3 + H2O ⇌ NH4+ + OH-. Understanding this equilibrium is crucial for calculating the concentration of hydroxide ions in the solution.

pH, pOH, and Their Relationship

pH and pOH are measures of the acidity and basicity of a solution, respectively. They are related by the equation pH + pOH = 14 at 25°C. The pH is calculated from the concentration of hydrogen ions (H+), while pOH is derived from the concentration of hydroxide ions (OH-). This relationship is essential for determining the pH and pOH of the ammonia solution.

Equilibrium Constant (Kb) for Ammonia

The base dissociation constant (Kb) quantifies the strength of a weak base like ammonia in water. For ammonia, Kb is approximately 1.8 x 10^-5. This constant is used in the equilibrium expression to calculate the concentration of OH- ions in the solution, which is necessary for finding the pH and pOH values.