The density of concentrated #"HCl"# is #"1.18 g/cm"^3#.
Assume that you have #"1 dm"^3# of conc. #"HCl"#.
Then
#"Mass of HCl solution" = 1000 color(red)(cancel(color(black)("cm"^3 "HCl solution")))× "1.18 g HCl solution"/(1 color(red)(cancel(color(black)("cm"^3 "HCl solution")))) = "1180 g HCl solution"#
#"Mass of HCl" = 1180 color(red)(cancel(color(black)("g HCl solution"))) × "36 g HCl"/(100 color(red)(cancel(color(black)("g HCl solution")))) = "425 g HCl"#
#"Moles of HCl" = 425 color(red)(cancel(color(black)("g HCl"))) × "1 mol HCl"/(36.46 color(red)(cancel(color(black)("g HCl")))) = "12 mol HCl"#
∴ The molarity of the solution is 12 mol/L.
Note: The answer can have only 2 significant figures, because that is all you gave for the concentration of the acid.
