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Go to a listing of many types of acid base problems and their solutions

Important note: all constants refered to: K_{c}, K_{w}, K_{a}, and K_{b} are temperature-dependant. All discussions are assumed to be at 25 °C, i.e. standard temperature.

Basic Information

1) Weak bases are less than 100% ionized in solution.

2) Ammonia (formula = NH_{3}) is the most common weak base example used by instructors.

The following equation describes the reaction between ammonia and water:

NH_{3}+ H_{2}O <==> NH_{4}^{+}+ OH¯

Note that it is an equilibrium condition.

The equilibrium constant for this reaction is written as follows:

K_{c}= ( [NH_{4}^{+}] [OH¯] ) / ( [NH_{3}] [H_{2}O] )

However, in pure liquid water, [H_{2}O] is a constant value. To demonstrate this, consider 1000 mL of water with a density of 1.00 g/mL. This 1.00 liter (1000 mL) would weigh 1000 grams. This mass divided by the molecular weight of water (18.0152 g/mol) gives 55.5 moles. The "molarity" of this water would then be 55.5 mol / 1.00 liter or 55.5 M.

The solutions studied in introductory chemistry are so dilute that the "concentration" of water is unaffected. So 55.5 molar can be considered to be a constant if the solution is dilute enough.

Moving [H_{2}O] to the other side gives:

K_{c}[H_{2}O] = ( [NH_{4}^{+}] [OH¯] ) / [NH_{3}]

Since the term K_{c} [H_{2}O] is a constant, let it be symbolized by K_{b}, giving:

K_{b}= ( [NH_{4}^{+}] [OH¯] ) / [NH_{3}]

This constant, K_{b}, is called the base ionization constant. It can be determined by experiment and each base has its own unique value. For example, ammonia's value is 1.77 x 10¯^{5}.

From the chemical equation above, it can be seen that NH_{4}^{+} and OH¯ concentrations are in the molar ratio of one-to-one. This will have an important consequence as we move into solving weak acid poblems.

Go to Solving K_{b} Problems: Part One

Go to Solving K_{b} Problems: Part Two

Go to Solving K_{b} Problems: Part Three

Go to a listing of many types of acid base problems and their solutions