When a solution of 6m hcl is made from a stock of concentrated hcl by adding an equal volume of water, why is the concentration of the solution only an approximate concentration?

Answer:-

HCl is a volatile acid. It easily evaporates at room temperature. So the concentrated HCl which is supposed to be 12 M is no longer 12 M since some of the HCl has already escaped as vapor.

Hence when a solution of 6M HCl is made from a stock of concentrated HCl by adding an equal volume of water, the concentration of the solution is only an approximate concentration since the concentration of the stock concentrated solution is also approximate.