Molarity, or molar concentration, is a measure of the amount of solute in a particular solution and is reported as moles per liter. Ethyl alcohol, or ethanol, can combine with water to form a solution. To identify the molarity of this solution, the amount of ethyl alcohol must be determined. Unlike many molarity problems involving solid solutes, ethanol is a liquid and the initial amount added to water is not expressed in terms of grams. Therefore, you must use other known properties of ethanol to determine the mass, in grams, of ethanol in a solution of water.
Measure out a specific amount of ethanol into a beaker. For example, pour 10 mL of ethanol into a beaker.
Calculate the grams of ethanol in the measured amount using the known density of ethanol. The Material Safety Data Sheet for ethanol reports the accepted density of ethanol as 0.790 g/cm^3. Density is expressed as mass per volume, and 1 cubic centimeter is equal to 1 milliliter. Therefore, the amount of ethanol, in grams, can be determined by multiplying the volume of ethanol by its density.
10 mL × 0.790 g/cm^3 = 7.9 g of ethanol
Determine the molar mass of ethanol. The molar mass is the sum of the molar masses of each individual atom of the ethanol molecule, which is composed of 2 carbon, 6 hydrogen and 1 oxygen atom. The molar mass of ethanol is then calculated as 46 g/mol.
Divide the amount, in grams, by the molar mass to calculate the number of moles of ethanol. 7.9 g/ 46 g/mol = 0.17 moles of ethanol
Add water to the ethanol and measure the volume of the resulting solution. For the example, the water and ethanol combine, forming a solution with a volume of 250 mL.
Divide by the conversion factor for milliliters to liters. The example solution contains 0.17 moles of ethanol in 250 mL of solution. Molarity is expressed in moles per liter and there are 1000 mL in 1 L. To convert, you divide 250 ml by 1000 ml/L therefore, there are 0.17 moles per 0.25 L.
Determine the molarity in terms of moles per liter. The previous step identified 0.17 moles of ethanol per 0.25 liters of solution. Setting up a ratio and solving for the unknown number of moles identifies 0.68 moles of ethanol per 1 liter of solution. This results in a molarity of 0.68 mol/L, or 0.68 M.
0.17 mol / 0.25 L = x mol/ L
x = 0.68 mol/L