How to Calculate the pH of Ammonia Water Using KB

HCl will lower the pH of water.
••• Ableimages/Digital Vision/Getty Images

Ammonia (NH3) is a gas that readily dissolves in water and behaves as a base. The ammonia equilibrium is describe with the equation NH3 + H2O = NH4(+) + OH(-). Formally, the acidity of the solution is expressed as pH. This is the logarithm of the concentration of hydrogen ions (protons, H+) in the solution. Base dissociation constant (Kb) is defined as Kb = [NH4+][OH-]/[NH3]. (Brackets refer to the molar concentration of ions or molecules in the solution.) Kb is a constant at a given temperature and is usually tabulated at 25 degrees Celsius. The Kb value for ammonia is 1.8E-5 (the notation “E-5” means “ten in power -5”).

    Multiply the Kb value by the total ammonia concentration and the number 4. The total concentration is the sum of [NH4+] and [NH3] in the solution. This concentration must be known or given in order to calculate pH. For example, the concentration is equal to 0.1 molar. Then the value in this Step should be calculated as 1.8E-5 x 0.1 x 4 = 7.2E-6.

    Take the square root of the value obtained in Step 1. In the example, it is sqrt(7.2E-6) = 2.683E-3. (Note that this result is rounded to thousandths.)

    Subtract the Kb value from the number obtained in Step 2 and then divide the result by 2 to calculate the concentration of the hydroxide ions (OH-). In our example, [OH-] = (2.683E-3 - 1.8E-5) /2 = 1.333E-3 molar.

    Divide the magnitude 1E-14 by the concentration of hydroxide ions (Step 3) to calculate the concentration of the protons [H+]: [H+] = 1E-14 / [OH-]. In our example, [H+] = 1E-14 / 1.333E-3 = 7.502E-11.

    Take the logarithm (with the base 10) of the proton concentration (Step 4), and multiply the result by -1 to calculate pH. In this example, pH = -1 x log(7.502E-11) = 10.12.

Related Articles

How to Find Hydroxide Ion Concentration
How to Calculate Percent Dissociation
How to Calculate Titratable Acidity
How Is pH Calculated?
How to Calculate End Point
How to Calculate the Concentration of Ions in a 0.010...
How to Calculate Solubilities
How to Determine pH From pKa
How to Calculate 2nd Equivalence Points
How to Calculate HCO3 From CO2
How to Calculate PH of Buffer Solutions
Sulfuric Acid & Chlorine Bleach Reaction
How to Find the PKA of a Weak Acid
How to Calculate Alkalinity After Titration
How to Calculate the K Value on a Titration Graph
How to Calculate Hydrogen Ion Concentration
How to Do Titration Calculations
How to Convert Kilojoules to Kilocalories
How to Find the X Intercept of a Function
How to Convert Micromolar to PPM