How a titration curve is affected when a poorly soluble salt is formed?
How a titration curve is affected when a poorly soluble salt is formed?


If by "I think that titration curve don't be affected" you mean that the graph of pH vs volume of strong base will not be affected by the precipitations, then you are right. This is because the species that form the precipitate are not part of any other equilibrium reaction.


  how to write a mass balance expression to take account into the precipitation of BA?


In general terms, the mass balance (really the amount balance) can be expressed in concentrations if the system is homogeneous (all in solution). For example, it could look like this:

$$[E] + [F] + [G] = \mathrm{constant}$$

If there is a solid species $H$ that you have to consider, use an expression based on amount of substance:

$$n_E + n_F + n_G + n_H= \mathrm{another\ constant}$$

If you want to, you can express parts of this as concentration again:

$$([E] + [F] + [G]) \cdot V_\mathrm{solution} + n_H= \mathrm{another\ constant}$$