Solutions: Calculating Concentrations including Dilution

Question

Calculate the concentration (mole fraction) of KCl in a solution prepared by dissolving 12.00 kg of KCl in 250.0 mL of water at 20 degrees celsius.

Answer 1

Mole fraction KCl = moles KCl / (moles KCl + moles H₂O)

molar mass KCl = 74.55 g / mol

molar mass H2O = 18.02 g / mol

Density of H2O @ 20ºC = 0.9982 g / ml

moles KCl present = 12.00 kg x 1000 g / kg x 1 mol / 74.55 g = 160.96 moles KCl

moles H2O present = 250.0 ml x 0.9982 g / ml x 1 mol / 18.02 g = 13.85 moles H2O

Total moles = 160.96 moles KCl + 13.85 moles H2O = 174.8 moles

Mole fraction KCl = 160.96 / 174.8 = 0.9208