±Units of Concentration: Molality and Mole Percent One of the many important pro
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±Units of Concentration: Molality and Mole Percent
One of the many important properties of solutions is concentration—which is the quantity of solute contained in a given quantity of solution.
The concentration of solution can be expressed in the following ways:
Molarity is the concentration of chemical solutions. It is the number of moles of solute dissolved in a liter of solution. It is denoted by M, and the unit is moles per liter, which can be written as molL.
Molality of a solution is the amount of solute divided by the mass of the solvent (not the mass of the solution). It is also defined as the number of moles of solute dissolved in one kilogram of solvent. It is denoted by m, which can be written as moles solute1 kg solvent.
Mole percent of a solute is the ratio of the moles of the solute in a solution to the moles of the solution reported as a percent (%). It represents the number of moles of a substance in a solution as a percentage of the total number of moles in the solution, which can be solved with the equation:
mole percent=moles solutemoles solution×100
The difference between molarity and molality
Molarity is the concentration of chemical solutions. It is the number of moles of solute dissolved in a liter of solution. It is denoted by M, and the unit is moles per liter, which can be written as molL.
molarity of a solution= moles of soluteliters of solution
Molality of a solution is the amount of solute divided by the mass of the solvent (not the mass of the solution). It is also defined as the number of moles of solute dissolved in one kilogram of solvent. It is denoted by m, which can be written as moles of solute1 kg of solvent.
molality= number of moles of solutemass of solvent =molal (moles/kilogram)
Even though molarity and molality are both measures of concentration of solutions, there is a difference. Molarity is the ratio of moles of solute to mass of the solution, whereas the molality is the ratio of moles of solute to the mass of the solvent.
Part A
A 3.50 g sugar cube (sucrose: C12H22O11) is dissolved in a 350 mL teapot containing 80 C water (density of water at 80 C= 0.975 g/mL). What is themolality of the sugar solution?
Express the molality with the appropriate units.
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Mole Percent
Mole percent is the number of moles of solute per moles of solution, where the number of moles of solution is equal to the number of moles of solute plus the number of moles of solvent:
mole percent= moles solutemoles solute+moles solvent×100=moles solutemoles solution×100
You should notice that you need to convert both the quantity of solute and the quantity of solvent to moles before solving for mole percent.
Part B
Calcium chloride, CaCl2, is commonly used as an electrolyte in sports drinks and other beverages, including bottled water. A solution is made by adding 7.50 gof CaCl2 to 60.0 mL of water at 25C. The density of the solvent at that temperature is 0.997 g/mL. Calculate the mole percent of CaCl2 in the solution.
Express your answer as a percent to three significant figures.
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Explanation / Answer
Part A :
moles of sucrose = g/molar mass = 3.50/342.2965 = 0.0102 mols
mass of solvent = volume x density = 350 x 0.975 = 341.25 g = 0.34125 kg
molality of solution = moles/kg of solvent = 0.0102/0.34125 = 0.03 m
Part B.
moles of CaCl2 = 7.50/110.98 = 0.0676 mols
moles of water = 60 x 0.997/18.02 = 3.32 mols
Thus, moles percent of CaCl2 = 100 x 0.0676/(0.0676 + 3.32) = 1.995%
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