Lewis base refers to substances that can donate a lone pair of electrons(e.g OH^-, Cl^- etc) or a pair of nonbonding electrons.(eg NH3, H20 etc). A Lewis base is therefore an electron-pair donor. They are easily recognized because they often have lone pairs or are negatively charged species.
If we look at the products, Cl- is a Lewis base because it is a lone pair donor. We see this more clearly because the specie is negatively charged.
For a gaseous sample, there is an inverse relationship between the pressure and the volume. If we consider the gas as an ideal gas, we can find the final volume using Boyle's law.