Percent ionization can be used to quantify the extent of ionization of an acid i

Question

Percent ionization can be used to quantify the extent of ionization of an acid in solution and is defined by the following formula for the acid HA: Percent ionization = [HA] ionized/[HA] initial times 100%. Percent ionization increases with increasing K_a. Strong acids, for which K_a is very large, ionize completely (100%). For weak acids, the percent ionization changes with concentration. The more diluted the acid is, the greater percent ionization. A convenient way to keep track of changing concentrations is through what is often called an I.C.E table, where I stands for "Initial Concentration, " C stands for "Change, " and E stands for "Equilibrium Concentration." To create such a table, write the concentrations of reactant (s) and product(s) across the top, creating the columns, and write the rows I.C.E on the left-hand side. Such a table is shown below for the reaction: A + B rightharpoonoverleftharpoon AB [A] [B] [AB] Initial (M) Change (M) Equilibrium (M) A certain weak acid, HA, has a K_a. value of 3.2 times 10^-7. Calculate the percent ionization of HA in a 0.10 M solution. Calculate the percent ionization of HA in a 0.010 M solution. Express your answer to two significant figures, and include the appropriate units.

Explanation / Answer

The dissociation reaction taking place is:

HA ----> H+ + A-

Initial 0.1 0 0

Eqb 0.1-x x x

Ka = ( [H+][A-] )/[HA] = x2/(0.1-x) = 3.2*10-7

Solving we get:
x = 0.000178

So, % ionization = (x/0.1)*100 = 0.178%

Similarly proceeding as above, when initial conc is 0.01 M, we get x = 0.0000564, and % ionization = 0.0564%

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