Commercial grade hydrochloric acid is typically sold as an aqueous solution that is 39.0% HCl by mass and has a density at 20 °C of 1.20 g/mL. Determine the molality of the HCl solution.

Calculate the mass of HCl in 1 liter of solution. Since the solution is 39.0% HCl by mass, this means that in 100 g of solution, there are 39 g of HCl. Use the density of the solution (1.20 g/mL) to find the mass of 1 liter of solution: \( \text{mass of solution} = 1.20 \text{ g/mL} \times 1000 \text{ mL} = 1200 \text{ g} \). Therefore, the mass of HCl in 1 liter of solution is \( 0.39 \times 1200 \text{ g} = 468 \text{ g} \).