How do I calculate the grams of a product after a reaction?
The concentration $c$ of a solution is the molar quantity of the solute $n$ devided by the volume of the solution $V$:

$\begin{equation}
c = \frac{n}{V}
\end{equation}$

as you state, for example for HCl 2.5 mol/L (or, equivalent, 2.5 M).  Inversely, if you know volume and concentration of your solution, you may access the molar quantity of your solute.  Actually, there is such an equation for each of the solutions, indepent from each other; you may index the variables accordingly, too.

Yet watch, there is a catch: mixing the two solutions together increases the total volume, and simultaneously decreases the concentration of the specis in question -- without altering the molar quantity of $\ce{Na+}$, nor $\ce{Cl-}$ ions present.  Hence "2.5 M of NaCl" (as measure of concentration) in the solution in common is incorrect.

Because of the initially asked question, one of the solutions contains the limiting reagent.  Figure out which one -- this includes the stoichiometry in the equation of chemical reaction. Knowing the molecular amount $n$ of it, your are able to determine the amount of sodium chloride formed; initially as moles, later (via molecular mass) in grams.