Question

Molarity of H₂SO₄ is 18 M. Its density is 1.8 g/ml. Hence molality is ______.

Options

• 36

• 200

• 500

• 18

Answer 1

Molarity of H₂SO₄ is 18 M. Its density is 1.8 g/ml. Hence molality is 500.

Explanation:

Using the density (1.8 g/mL):

Mass of solution = Volume × Density

= 100 mL × 1.8 g/mL

= 1800 g

We know the molarity is 18 M, meaning there are 18 moles in our 1 Liter. The molar mass of H₂SO₄ is 98 g/mol.

Mass of solute = Moles × Molar mass

Mass of solute = 18 mol × 98 g/mol

= 1764 g

Subtract the solute mass from the total solution mass:

Mass of solvent = Mass of solution − Mass of solute

Mass of solvent = 1800 g − 1764 g 

= 36 g

Molality is defined as moles of solute per kg of solvent:

`m = "Moles of solute"/"Mass of solvent (kg)"`

`m = (18  mol)/(0.036  kg)`

= 500 m