Commercial grade hcl solutions are typically 39.0% (by mass) hcl in water. determine the molality of the hcl, if the solution has a density of 1.20 g/ml

Molality of the solution is defined as the number of moles of a substance dissolved divided by the mass of the solvent:

Molality = number of moles / solvent mass

From the concentration of 39% (by mass) of HCl in water, we construct the following  reasoning:

in 100 g solution we have 39 g hydrochloric acid (HCl)

number of moles  = mass / molecular weight

number of moles of HCl = 39 / 36.5 = 1.07 moles

solvent (water) mass = solution mass - hydrochloric acid mass

solvent (water) mass = 100 - 39 = 61 g

Now we can determine the molality:

molality = 1.07 moles / 61 g = 0.018

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