Question 17.2: Calculating Concentrations of Species in a Weak Acid Solutio...
Calculating Concentrations of Species in a Weak Acid Solution Using K_{a} (Approximation Method)
What are the concentrations of nicotinic acid, hydrogen ion, and nicotinate ion in a solution of 0.10 \mathrm{M} nicotinic acid, \mathrm{HC}_{6} \mathrm{H}_{4} \mathrm{NO}_{2}, at 25^{\circ} \mathrm{C} ? What is the \mathrm{pH} of the solution? What is the degree of ionization of nicotinic acid? The acid-ionization constant, K_{a}, was determined in the previous example to be 1.4 \times 10^{-5}.
PROBLEM STRATEGY
You follow the three steps for solving equilibrium problems that were introduced in Example 15.7. In the last step, you solve the equilibrium-constant equation for the equilibrium concentrations. The resulting equation is quadratic, but because the equilibrium concentration of a weak acid is usually negligibly different from its starting value, the equation simplifies so that it involves only the square of the unknown, which is easily solved by taking the square root. (You will need to check that this assumption is valid.)
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