Question

What is the molality (m) of 4.26% HCI by weight of an aqueous hydrochloric acid solution? Be sure to report your answer in the correct number of significant figures and use the correct units.

Answer 1

Let mass of solution be 1 Kg = 1000 g

mass of HCl = 4.26 % of mass of solution

= 4.26*1000.0/100

= 42.6 g

mass of solvent = mass of solution - mass of solute

mass of solvent = 1000 g - 42.6 g

mass of solvent = 957.4 g

mass of solvent = 0.9574 Kg

Molar mass of HCl,

MM = 1*MM(H) + 1*MM(Cl)

= 1*1.008 + 1*35.45

= 36.458 g/mol

mass(HCl)= 42.6 g

use:

number of mol of HCl,

n = mass of HCl/molar mass of HCl

=(42.6 g)/(36.46 g/mol)

= 1.168 mol

m(solvent)= 0.9574 Kg

use:

Molality,

m = number of mol / mass of solvent in Kg

=(1.168 mol)/(0.9574 Kg)

= 1.22 molal

Answer: 1.22 molal