Why is Ma*Va=Mb*Vb still valid for weak acids/bases?
We normally titrate a weak acid by a strong base, or  weak base by a strong acid, to ensure that the reaction of titration is total (quantitative). We don't titrate weak acid by a weak base (and vice-versa).
I will calculate the equilibrium constant of the titration of  weak acid $\ce{HA}$ by a strong base $\ce{OH-}$:
 $$\ce{HA + OH- &lt;=&gt;A- + H2O}$$ The equilibrium constant is $$K=10^{(\mathrm{p}K_w -\mathrm{p}K_a)}$$ For common weak acids (acetic acid $\mathrm{p}K_a=4.7$ ): $$K&gt;&gt; 10^3$$
So, the equation of titration (neutralization) must be written:$$\ce{HA + OH- -&gt;A- + H2O}$$
At the equivalence point: $$n(\ce{HA})=n(\ce{OH-})$$