What mass of hcl is contained in 45.0 ml of an aqueous hcl solution that has a density of 1.19 g cm–3 and contains 37.21% hcl by mass?

When the concentration is expressed in percentage, that is the ratio of the mass of solute to the mass of the total solution. Since the given amount of solution is in terms of volume, let's use the density to find the mass. Note that 1 mL is just equal to 1 cm³.

45 mL * 1.19 g/cm³ = 53.55 g HCl solution

Mass Percent = Mass Solute/Mass Solution * 100
37.21% = Mass Solute/53.55 g * 100
Mass Solute = 19.93 g HCl