The percent by mass of 1,6-hexandiol (MM = 118.17 g/mol) in an aqueous solution is 33.3%. What is the molality of the 1,6-hexandiol solution?

Question

The percent by mass of 1,6-hexandiol (MM = 118.17 g/mol) in an aqueous solution is 33.3%. What is the molality of the 1,6-hexandiol solution

Answer 1

Let's start out with defining what % by mass means. A solution that is 33.3% by mass 1,6 hexandiol, means there are 33.3 g of this solute per 100 g of solution (% means parts per 100 parts).

Next, let's define what molality means. Molality (m) is defined as moles of solute (1,6 hexandiol) per kg of solvent (water), or moles of solute per 1000 g of solvent. So, to find the molality, we need to find moles of solute and kg of solvent:

Let us assume we have 1000 g of solution. That means we have 333 g of solute (per definition of 33.3%)

333 g 1,6 hexandiol x 1 mol / 118.17 g = 2.82 moles of 1,6 hexandiol

kg of water: 1000 g solution - 333 g solute = 667 g water = 0.667 kg water

MOLALITY = moles / kg = 2.82 mols / 0.667 kg = 4.23 m