= Weak acid
A <weak acid> establishes a <chemical equilibrium> between its proton-donating form and its <conjugate base> in <water>. For a neutral <monoprotic acid> $\mathrm{HA}$, the aqueous <chemical equation> is
$$
\mathrm{HA(aq)}+\mathrm{H_2O(l)}\rightleftharpoons\mathrm{H_3O^+(aq)}+\mathrm{A^-(aq)}.
$$
<Water> accepts a <proton> to become <hydronium>; $\mathrm A^-$ is the <conjugate base>. The double arrow denotes reversible exchange at <chemical equilibrium>, not equal populations. The <acid dissociation constant> and composition determine the <hydronium> <concentration>; substituting total <chemical acid> <concentration> into the <pH> formula is generally incorrect.
“Weak” describes the tendency to transfer a <proton> in a specified <solvent>, rather than how much <chemical acid> was dissolved. A <weak acid> can be concentrated, and a <strong acid> can be dilute. Even the same <weak acid> can have a large <weak acid dissociation fraction> after sufficient <dilution>, while its <acid dissociation constant> stays fixed at the same <temperature> and <standard state> conventions.<Chemistry 2e by Paul Flowers and others/14.3 Relative Strengths of Acids and Bases>[§14.3, acid ionization and concentration-dependent percent ionization]{ref}
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