Give the definition of a weak acid and provide the equation for the ionization of a weak acid in water.

Question

AndrewArni · Answer

Weak acid and base ionization reactions and the related equilibrium constants,  Ka and Kb. Relating Ka and Kb to pH, and calculating percent dissociation.

justjm · Answer

A weak acid is an acid that does not dissociate fully.

There are only a few strong acids (100% dissociation): HCl, HBr, HI, HClO4, HClO3, HNO3, and the diprotic H2SO4 [HF is NOT a weak acid because Flourine has the HIGHEST electronegativity]. If you have an acid (willing to donate a proton) that is not in that list, then it is most likely a weak acid.

Now that was a rather conventional definition of a weak acid. There is an actual way to calculate dissociation:

$ \% _{ion}=\frac{[HA]_{eq}}{[HA]_i} \times100$
A weak acid is defined to have percent dissociations less than 5% (this is known as the 5% rule).
The $K_a$ works likewise. The equilibrium constant $K_a$ is typically VERY low for weak acids. $pK_a$ is hence very high.

justjm · Answer

For equation, are you asking for the net-ionic?