Part D: Molality Unit Molality, m, is a ratio of moles of solute divided by kg o

Question

Part D: Molality Unit Molality, m, is a ratio of moles of solute divided by kg of solvent. This is kg of solvent, not solution. One advantage that the molality unit has is that it is not temper- ature dependent; volume is not in the denominator. Please answer the following questions involving the molality unit; show all of your calculations 17. A 0.20 molality K20 solution needs to be prepared for a Quality Control lab located in a manufacturing plant. How many grams of K2O will be needed when 268 g of solvent will be used to prepare the solution? How much solvent is needed, in units of g, to prepare a 0.030 molality solu- tion of K2O that will be prepared with 0.048 g of K20? 18.

Explanation / Answer

Q17

m = 268 g of solvent

m = 0.268 kg of solvent

molality = mol of solute/ kg of solvent

mol = molality * kg = 0.2*0.268 = 0.0536 mol of solute; K2O

mass = mol*MW = 0.0536*94.196 = 5.0489 g of K2O

Q18

m = 0.03 molal

mol = mass/MW = 0.048 / 94.196 = 0.00050957 mol

molal = mol / kg

kg = mol/molal = 0.00050957 / 0.03 = 0.016985 kg of solvent

mass = 0.016985*1000 = 16.985 g of solvent

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