A concentration of 2.7M is really 2.7 mol/L of solvent. The key to this problem is determine how many moles you have of HCl, because you can use this and the concentration to determine volume.
HCl has a molar mass of:
H - 1.00784 g/mol
Cl - 35.453 g/mol
Once you have the molar mass, you can use it to find tbe number of moles of HCl you are working with:
50g HCl * (1mol/36.46g) = 1.371mol
Then, you can use the original molarity to determine liters of solvent:
1.371mol / (2.7mol/L) = 0.507L of solvent
The concentration of each ion in this case is 2.7M, just like the overall concentration, because there is one H+ ion and one Cl- ion equivalent to every HCl. If the acid had been something like H3PO4, the concentration of the H+ ion would be 3x that of the acid overall.