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It is readily shown that when pH = pKa then there is 50% ionization.

For an acid, if the pH is greater than the pKa the percentage ionization will be

50%
Greater than 50%
Less than 50%

For a base, if the pH is greater than the pKa the percentage ionization will be

Greater than 50%
Less than 50%
50%

Frequently we do not need to know the exact percentage ionization in order to draw meaningful conclusions. Even if the calculation is exact there may be some uncertainty in the pKa and pH and hence in the calculated percentage ionization. The following values provide a good RULE of THUMB for estimating percent ionization for acids and bases.

You already know that when pH=pKa there is 50% ionization. The series 10, 1, 0.1 and 90, 99, 99.9 are easy to remember and little thought will tell you whether the %ionization is greater than or less than 50%.

An acidic drug, pKa 10, will be 1% ionized at pH 8

TRUE
FALSE

A basic drug, pKa 5, will be 99.9% ionized at pH 2

TRUE
FALSE

Chlorpropamide is an acidic drug with pKa = 5.0. Which of the following best describes the percentage ionization of chlorpropamide in the stomach at pH1

Unionized
About 50% ionized
Fully ionized
About 0.1% ionized
About 99.9% ionized

Chlorpropamide is an acidic drug with pKa = 5.0. Which of the following statements about the percentage ionization of chlorpropamide in blood at pH 7.4 is correct?

Between 99% and 99.9% ionized
Between 90% and 99% ionized
Over 99.9% ionized
Between 10% and 90% ionized
Between 1% and 10% ionized
Between 0.1% and 1% ionized
Less than 0.1% ionized

Select the CORRECT statement(s) about the percentage ionization of a base.

A basic drug, pKa 4, will be 10% ionized at pH 5
A basic drug, pKa 10, will be fully ionized at pH 4
A basic drug, pKa 9, will be 90% ionized at pH 10
A basic drug, pKa 5, will be 99.9% ionized at pH 2
A basic drug, pKa 5, will be 0.1% ionized at pH 10

Select the CORRECT statement(s) about the percentage ionization of an acid.

An acidic drug, pKa 5, will be 99% ionized at pH 7
An acidic drug, pKa 3, will be 99.9% ionized at pH 6
An acidic drug, pKa 7, will be fully ionized at pH 2
An acidic drug, pKa 8, will be 90% ionized at pH 7
An acidic drug, pKa 4 will be 50% ionized at pH 4

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