Calculate the concentration, given the percent dissociation

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Problem #1: What must be the molarity of an aqueous solution of NH3 if it is 4.1% ionized?

Solution:

Kb = ([NH4+] [OH¯]) / [NH3]

1.77 x 10-5 = [(0.041x) (0.041x)] / (x)

0.001681x2 = 1.77 x 10-5x

0.001681x2 - 0.0000177x = 0

Use this quadratic equation root solver.

x = 0.0105 M

Comment: note that the value for the Kb is not provided in the problem, but is required for the solution. The Kb for ammonia is available in many references soures.


Problem #2: Determine the molar concentration for an acetic acid (CH3COOH) solution known to be 3.5% dissociated. The Ka is 1.77 x 10-5

Solution:

Ka = ([H+] [CH3COO¯]) / [CH3COOH]

1.77 x 10-5 = [(0.035x) (0.035x)] / (x)

0.001225x2 = 1.77 x 10-5x

0.001225x2 - 0.0000177x = 0

Use this quadratic equation root solver.

x = 0.0144 M


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