Question 17.1: Determining Ka from the Solution pH Nicotinic acid (niacin) ...
Determining Ka from the Solution pH
Nicotinic acid (niacin) is a monoprotic acid with the formula \mathrm{HC}_{6} \mathrm{H}_{4} \mathrm{NO}_{2}. A solution that is 0.012 \mathrm{M} in nicotinic acid has a \mathrm{pH} of 3.39 at 25^{\circ} \mathrm{C}. What is the acidionization constant, K_{a}, for this acid at 25^{\circ} \mathrm{C} ? What is the degree of ionization of nicotinic acid in this solution?
PROBLEM STRATEGY
It is important to realize that when we say the solution is 0.012 \mathrm{M}, this refers to how the solution is prepared. The solution is made up by adding 0.012 \mathrm{~mol} of substance to enough water to give a liter of solution. Once the solution is prepared, some molecules ionize, so the actual concentration is somewhat less than 0.012 \mathrm{M}. To solve for K_{a}, you follow the three steps outlined in Chapter 15 for solving equilibrium problems.
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