Question

An aqueous solution is made by dissolving 18.5 grams of zinc sulfate in 421 grams of water. The molality of zinc sulfate in the solution is ____ m.

Answer 1

The formula for molality is written as below

Molality = moles of solute ⁄ mass of solvent in kg …..equation 1

In the question, water is the solvent and zinc sulphate (ZnSO4) is the solute.

From the data

Mass of solvent = 421g = 0.421kg

Mass of solute =18.5 g

Now we need to find the moles of solute

Moles of solute = mass of solute ⁄ molar mass of solute…..equation 2

Molar mass of solute (ZnSO4) is calculated as

= mass of Zn + mass of S + (4×mass of O)

= 65.38 + 32.065 + (4×16)

= 161.445gmol-1

Put the value of molar mass of solute from the above calculation and mass of solute from the data in equation 2.

=18.5 g ⁄ 161.445gmol-1

= 0.1146 mol

So the number of moles = 0.1146 mol

Put the value of number of moles of solute from the above calculation and the mass of solvent from the data in equation 1.

=0.1146mol ⁄ 0.421kg

=0.2721mol kg-1

So the molality of the given solution is 0.2721mol kg-1or 0.2721 m.