Question

What is the molality of an aqueous kbr solution with a mole fraction of kbr, x, equal to 0.245?

Answer 1

Molality is used to express concentration and it is:
molality = moles of solvent / kilogram solution

Now, lets assume there is one mole of solution. Then, there will be 0.245 moles of KBr and 0.755 moles of water. We multiply these molar fractions with the substances' molar masses to find the weight of one mole of solution.

Weight of one mole of solution = 0.245 * 119 + 0.755 * 18
Weight = 42.74 grams

Converting this weight to kilograms, this is 0.043 kg. We now multiply the moles of KBr by the kilograms of solution to find the molality:

molality = 0.245 / 0.043

The molality of the solution is 5.70 moles per kg