Question

A commonly purchased disinfectant is a 3.0% (by mass) solution of hydrogen peroxide (H2O2) in water. Assuming the density of the solution is 1.0 g/cm3 , calculate the molarity and molality of H2O2.

Answer 1

Concentration by mass is 3 % w/w.

Density of solution = 1 g/ ml

Let volume of solution be 1 liter or 1000 mL

So, mass of solution = 1 x 1000 g => 1000 g

Out of this 1000 g, 3 % is pure H2O2.

Hence, 30 grams is pure H2O2 and 1000-30 => 970 grams is water.

molar mass of Hydrogen Peroxide(H2O2) is 2 x 1 + 2 x 16 => 34 g mol-1

Hence, 30 grams of H2O2 is 30/34 =>15/17 moles => 0.882 moles

Volume of solution= 1 L

Molarity = (moles of solute)/ (litres of solution) =>0.882/1

Molarity of 3 % H2O2 solution = 0.882 M (molar)

Molality = (moles of solute)/(kilograms of solvent) => 0.882/0.97

Molality =  88.2/97 => 0.909 mol kg-1

Molality of 3 % H2O2 solution = 0.909 mol kg-1