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

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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. 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. 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. 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. 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.

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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.

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00:51

ICE Charts of Weak Bases

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.

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pH and pOH Calculations

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.

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