This page was constructed from content via the following contributor s and edited topically or extensively by the LibreTexts development team to meet platform style, presentation, and quality:. Learning Objectives Define a strong and a weak acid and base. Recognize an acid or a base as strong or weak.
The first K a refers to the first dissociation step:. This K a value is 4. The second K a is 4. The K a of acetic acid is [latex]1. What is the pH of a solution of 1 M acetic acid? This quadratic equation can be manipulated and solved. A common assumption is that x is small; we can justify assuming this for calculations involving weak acids and bases, because we know that these compounds only dissociate to a very small extent.
Therefore, our above equation simplifies to:. The same basic method can be used to determine the pH of aqueous solutions of many different weak acids and bases. An aqueous solution of a weak acid or base contains both the protonated and unprotonated forms of the compound, so an ICE table can be made and used to plug in concentrations into an equilibrium constant expression.
The ionization constant for the acid K a or base K b is a measure of how readily the acid donates protons or how readily a base accepts protons. Because you are calculating pH, you must solve for the unknown concentration of hydronium ions in solution at equilibrium. The first step in calculating the pH of an aqueous solution of any weak acid or base is to notice whether the initial concentration is high or low relative to 10 -7 M the concentration of hydronium and hydroxide ions in water due to the autoionization of water.
If the concentration of the acid or base is very close to or less than 10 -7 M, then the solution is considered dilute and additional steps must be taken to calculate pH.
You must first be familiar with equilibrium constant expressions and how to write them for a chemical reaction. Then, by making an ICE table , you can find unknown concentration values that can be plugged into this equilibrium expression.
What is the pH of 1. To start, you must find the initial concentration of acetic acid in the vinegar. Now make an ICE table, considering the ionization of acetic acid in water into acetate ion and hydronium ion. Because only solutes and gases are incorporated into the equilibrium expression, you can ignore the concentration of water a pure liquid in our calculations.
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