how to dilute 7.75g of conc. HCl in 1000cm3?
Are you asking how to dilute the HCl?
If you are intending to dilute using a volumetric flask then it is a good idea to add acid to water than water to acid, although it is not as crucial with conc HCl (Cf H2SO4). The dilution will still get warm.
My sequence would be to place approximately 500 ml of water in a 1 L beaker containing a stirrer bar.
Weigh out the 7.75 g of conc HCl into a clean dry beaker.
Transfer the conc HCl to the beaker containing the 500 ml water while stiring the water. Rinse the beaker containing the conc HCl into the 500 ml water. I would do this say 3 times.
Cover the beaker containing the solution (with watch glass or film) and allow to cool to room temperature.
Transfer the solution to a 1 L volumetric flask, and rinse the beaker with water (again 3 times).
Make up to 1 L mark with water.