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How to Find the pH of an Ammonia Solution

When you add a strong base to water, each base molecule attracts one proton (H+) from each molecule of water (H2O), leaving hydroxide (OH-). The base ammonia (NH3) and water (H2O), for example, become NH4+ and OH-. Chemists express the tendency of a base to do this as the base-dissociation constant (Kb) of the base. The larger the Kb value of a base, the more alkaline, or basic, the base is. Ammonia has a Kb of 1.8 x 10^-5. Knowing the Kb of ammonia will help you find the pH of an ammonia solution.

Things You'll Need

Pencil
Paper
Scientific calculator

Instructions

- 1
  Write the base-dissociation constant (Kb) of ammonia. For example, write, "Kb = 1.8 x 10^-5."
- 2
  Write the concentration in moles per liter (M) of a laboratory or commercial ammonia solution. The concentration of a laboratory or commercial ammonia solution is 14.8 moles per liter. For example, write "14.8 M."
- 3
  Find the product of the Kb of ammonia and the concentration in moles per liter you have written using a scientific calculator. For example, 1.8 x 10^-5 x 14.8 = 2.664 x 10^-4.
- 4
  Take the square root of your answer using your calculator. For example, (2.664 x 10^-4)^-1/2 = 0.016321.
- 5
  Take the negative log of your answer using your calculator. For example, -log(0.016321) = 1.7872.
- 6
  Subtract your answer from 14. For example, 14 - 1.7872 = 12.2128. The pH of the ammonia solution is approximately 12.2, which is strongly basic.

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