The reversible double-reaction arrow is used in weak acid-base dissociation
reactions.
Being able to identify weak acids and bases will really help you on test day. No
one memorizes the list of weak acids and weak bases because there are tens of
thousands of them. The way to identify a weak acid or weak base is first to
recognize whether a compound is acidic or basic, and then know that if it isn’t in
the set of the strong acids or strong bases, it must be weak.
Calculating pH for Weak Acids or Base Solutions
Weak acids and bases partially, reversibly dissociate. For this reaction, a
dissociation constant, Ka for acids and Kb for bases, must be used to calculate the
pH of a solution of weak acid or base.
Example: Given that KaHF = 7 × 10−4, what is the pH of 1.5 M HF(aq)?
Solution: First, write the balanced chemical equation.
HF(aq) H+(aq) + F−(aq)Second, write the equilibrium expression, so we can use
KaHF.KaHF = Third, write the algebraic expression (plug in numbers
where you have them, and plug in letters where you
don’t).