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General Chemistry Principles, Patterns, and Applications, 2011

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Equation 16.38<br />

CCH 3CO2H = [CH 3CO2H ] + [CH 3CO2-]<br />

As we shall see shortly, if we know the analytical concentration <strong>and</strong> the Ka, we can calculate the actual<br />

values of [CH3CO2H] <strong>and</strong> [CH3CO2 − ].<br />

The equilibrium equations for the reaction of acetic acid <strong>and</strong> ammonia with water are as follows:<br />

Equation 16.39<br />

Ka = [H +][CH 3CO2-][CH 3CO2H ]<br />

Equation 16.40<br />

Kb = [NH 4+][OH-][NH 3]<br />

where Ka <strong>and</strong> Kb are the ionization constants for acetic acid <strong>and</strong> ammonia, respectively. In addition to the<br />

analytical concentration of the acid (or the base), we must have a way to measure the concentration of at<br />

least one of the species in the equilibrium constant expression to determine the Ka (or the Kb). There are<br />

two common ways to obtain the concentrations: (1) measure the electrical conductivity of the solution,<br />

which is related to the total concentration of ions present, <strong>and</strong> (2) measure the pH of the solution, which<br />

gives [H + ] or [OH − ].<br />

Example 6 <strong>and</strong> Example 7 illustrate the procedure for determining Ka for a weak acid <strong>and</strong> Kb for a weak<br />

base. In both cases, we will follow the procedure developed inChapter 15 "Chemical Equilibrium": the<br />

analytical concentration of the acid or the base is the initial concentration, <strong>and</strong> the stoichiometry of the<br />

reaction with water determines the change in concentrations. The final concentrations of all species are<br />

calculated from the initial concentrations <strong>and</strong> the changes in the concentrations. Inserting the final<br />

concentrations into the equilibrium constant expression enables us to calculate the Ka or the Kb.<br />

E X A M P L E 6<br />

Electrical conductivity measurements indicate that 0.42% of the acetic acid molecules in a 1.00 M solution<br />

are ionized at 25°C. Calculate K a <strong>and</strong> pK a for acetic acid at this temperature.<br />

Given: analytical concentration <strong>and</strong> percent ionization<br />

Asked for: K a <strong>and</strong> pK a<br />

Strategy:<br />

A Write the balanced equilibrium equation for the reaction <strong>and</strong> derive the equilibrium constant<br />

expression.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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