Question

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 1

Are you speaking of the stock solution as the solution taken from a 2.5 L bottle of concentrated HCl as sold by commercial supply houses? Probably so. Those solutions are made by dissolving gaseous HCl in water to make a 35-37% solution of HCl. It is approximately 12 M and you work from there; however, approximate means just that. Therefore, any dilutions you make from that will be approximate also.