The density of concentrated "HCl"HCl is "1.18 g/cm"^31.18 g/cm3.
Assume that you have "1 dm"^31 dm3 of conc. "HCl"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.
