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How to Calculate the Kb for a Base If It Is Ionized

The concept of "equilibrium" --- the idea that processes occur in back-and-forth continuums rather than in only one direction or another --- is a fundamental principle of chemistry. This idea is particularly important when it comes to acid-base chemistry. When you dissolve an acid or base in water, for example, most of its ions --- hydrogen for an acid or hydroxide for a base, plus another ionic component, depending on the specific acid or base --- "dissociate," or come apart, but some flux back and forth, joining back together and then dissociating again. The "equilibrium constant" --- "Kb" for a base --- represents the rate at which this flux occurs.

Calculator

Instructions

- 1
  Write the equilibrium expression for the ionization of your base in the following form: Kb= [concentration of ions]/[concentration of solution].
  
  For the dissociation of ammonium hydroxide, or NH4OH --> NH4(+) + OH(-), this expression is as follows:
  
  Kb = [NH4+][OH-]/[NH4OH]
- 2
  Plug each component's concentration into the equation, making sure you input the correct value into each space. For example, if your chemistry textbook or problem informs you that dissolving 1.031 M of ammonium hydroxide in water results in an OH- concentration of .0038 M, and an NH4+ concentration of .0048 M, you would input your values as follows:
  
  Kb = [.0038][.0048] / [1.031]
- 3
  Complete your calculation and express your answer in scientific notation. For the example calculation, where KB = (.0038 x .0048) / 1.031, this answer is .0000178. Count the number of places your decimal is moved left from "ones" position --- in the case of the example, this is five --- and express the value in terms of the smallest number that is greater than one. The number ".0000178" is "1.78 x 10^-5" in scientific notation.

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