Concentrations of Molecular Solutes

Let's consider a simple molecule, ethanol or CH3CH2OH, dissolved in water.

We measure the concentration of a molecular substance in a solution through molarity. Molarity (M) is the number of moles of the molecule per liter of the solution.

What is molarity of a solution made by adding 10 g of ethanol in 100 mL of an aqueous solution?

(10 g CH3CH2OH)/(46 g/mol) = 0.22 mol

(0.22 mol)/(0.1 L) = 2.2 M

Ethanol is very soluble in water. It interacts with water through London forces, dipole-dipole interactions, and hydrogen bonding.

The extent of hydrogen bonding between water and ethanol is greater than between ethanol molecules in pure ethanol. This is because the oxygen in ethanol can accept 2 hydrogen bonds but there is only 1 hydrogen atom that can form hydrogen bonds. Water can accept 2 hydrogen bonds at each oxygen atom and can donate 2 hydrogen bonds with its hydrogen atoms. The hydrogen bonding in pure ethanol is shown below.

Both ethanol and water are liquids. We might expect that a mixture of the two would have a volume equal to the sum of the volume of ethanol and the volume of the water.

Pure ethanol has a lower density than water, 0.789 g/mL at 25 deg as opposed to 1.000 g/mL for pure water. The density of an ethanol solution in water will have a density lower than water. The exact density depends on the ratio of ethanol to water.

Because of the interactions between molecules, a mixture of ethanol and water has a volume that is a little lower than the sum of the volumes of the two components.

Concentrations of Ionic Solutes

Now consider a salt, such as NaCl, in water. Salts dissociate in water so for a solution made from 10 g of NaCl in 100 mL of solution:

There are double the number of particles than the number of moles per liter of NaCl. Remember that it is the total number of particles that affects the colligative properties such as the freezing point depression or boiling point elevation of a solution.

The sodium cations and chloride anions interact very stongly with water. The oxygen atom of water molecules acts as a Lewis base and coordinates to the Lewis acidic sodium cation in donor-acceptor bonds.

Solutions of sodium chloride have a density that is very close to that of water. The 1.7 M NaCl solution has a density of 1.069 g/mL as compared to 1.000 g/mL for pure water at 25 deg.

Density Changes with Concentration

Crystalline sodium chloride, NaCl(s) has a higher density than water at 2.165 g/mL. The density of any NaCl solution will be greater than that of pure water but, as we saw above, the density is close to that of pure water.

The density of a sodium chloride solution increases with the concentration of the salt.

Salt Institute

Professor Patricia Shapley, University of Illinois, 2011