Question

Concentrated aqueous HCl has a molarity of 11.7 mol/L and a density of 1.20 g/cm³. What is the molality of HCl in the solution?

Answer 1

Let volume be 1 L

volume , V = 1 L

we have below equation to be used:

number of mol ,

n = Molarity * Volume

= 11.7*1

= 11.7 mol

volume , V = 1 L

= 1*10^3 mL

density, d = 1.2 g/mL

we have below equation to be used:

mass = density * volume

= 1.2 g/mL *1*10^3 mL

= 1200.0 g

This is mass of solution

Molar mass of HCl = 1*MM(H) + 1*MM(Cl)

= 1*1.008 + 1*35.45

= 36.458 g/mol

we have below equation to be used:

mass of HCl,

m = number of mol * molar mass

= 11.7 mol * 36.458 g/mol

= 4.266*10^2 g

This is mass of solute

mass of solvent = mass of solution - mass of solute

= 1200 - 426.5586

= 773.4414 g

= 0.77344 Kg

mass of solvent = 0.77344 Kg

we have below equation to be used:

Molality,

m = number of mol / mass of solvent in Kg

=(11.7 mol)/(0.77344 Kg)

= 15.13 molal

Answer: 15.1 molal