Untitled - Kelly Walsh High School

Solutions 175
Don’t Forget!
Don’t Forget!
In preparing a molar solution, the correct number of moles of solute (commonly
converted to grams using the molar mass) is dissolved and diluted to the
required volume.
Molality
Molality (m) is the moles of solute per kilogram of solvent.
m = moles solute/kg solvent
Notice that it is kilograms of solvent, not solution. In the other concentration
units, it has been the mass or volume of the solution. In molal solutions, it is the
mass of the solvent.
Ethylene glycol (C 2H 6O 2) is in antifreeze. Determine the molality of ethylene glycol
in a solution prepared by adding 31.0 g of ethylene glycol to 200.0 g of water.
Molality a 31.0 g C2H6O2 g
ba1000
200.0 g H2O 1kg ba1 mol C2H6O2 b 2.50 m C2H6O2 62.1 g C2H6O2 It is the kilograms of the solvent in the denominator and not the solution.
Mole Fraction
The mole fraction, X, is the moles of solute divided by the total number of
moles of solution.
X moles solute/total moles of solution
Suppose you have a solution containing 0.50 mol of sodium chloride in 2.0 mol
of water. The mole fraction of sodium chloride would be:
XNaCl moles NaCl/(moles NaCl moles H2O) 0.50/(0.50 2.00) 0.20