Course Outline


Aqueous Solutions



 

After completing this unit you should be able to:

  • understand the observed properties of water in terms of the behavior of molecules
  • describe an aqueous solution at the level of molecules and ions
  • understand and use molarity, concentration expressed as moles of solute per liter of solution

Water

As we proceed further with our study of chemistry we must begin to 'see' matter in our mind's eye, not as it appears in the macroscopic world, but as it would appear if we could see atoms and molecules.  For example, you should be able to calculate that a glass of water (360 mL) contains 1.20 x 1025 molecules.  Imagine that as you stare at a glass of water you can zoom in on a small volume to higher and higher magnification until you can see a myriad of discrete water molecules, all with the same shape and size, touching each other and occupying their own little portion of the total volume.  The molecules are in constant motion, sliding past each other and changing places as they move.

The macroscopic properties that we associate with a liquid are due to this behavior of the molecules.  The liquid has a defined volume because each molecule has a defined volume, and there is essentially no space between the molecules.  This also means it is very difficult to compress a liquid.  A liquid flows to adopt the shape of the container, or can be forced through a tube, because the molecules can slide past each other.  We make use of these properties in hydraulic systems, the brakes on a car for example.

If we cool down our glass of water we would see the molecules move slower and slower relative to each other, until finally they stop sliding past each other.  We now have solid ice, and although the molecules continue to vibrate, each one occupies its own fixed place in the crystal lattice.  This lack of relative motion is what gives a solid its macroscopic characteristics - a fixed, rigid shape.  We can carve the ice into an elaborate sculpture, and enjoy its form until it warms up and melts into a liquid, losing its shape as the molecules resume sliding past each other.

The molecules in the liquid are held close to each other by forces of attraction (intermolecular forces) that are much weaker than the covalent bonds which hold the atoms together in molecules.  If we heat our sample of water, we would see the molecules move faster and faster, until they have so much kinetic energy that individual molecules can escape the grip of the intermolecular forces.  They escape from the surface of the liquid and move off independently into the gas phase.

Most of the volume of a gas is empty space.  The molecules move in straight lines unless they collide with another molecule, or with the walls of the container, in which case they bounce off and continue in a new direction until the next collision.  Gases have a low density and are easily compressed because of all the empty space between the molecules, and the motion of the molecules causes the gas to occupy the entire volume of a closed container or escape from an open one.

Dilute Aqueous Solutions

Many molecular compounds, sucrose (table sugar, C12H22O11) for example, dissolve in water to form solutions.  In the solid, sucrose molecules are lined up rigidly, one beside the next in all three dimensions.  When the solid dissolves, individual sucrose molecules become surrounded by water molecules and drift away from the other sucrose molecules.  Eventually all of the sucrose molecules are far apart from each other, moving independently, surrounded by water molecules, and evenly distributed.  When there are very many more water molecules than sucrose molecules, this is called a dilute aqueous solution .  Water is the solvent and sucrose is the solute.  Individual sucrose molecules are unchanged by being dissolved.

Many ionic compounds also dissolve in water to form solutions.  In this case it is the individual ions that are surrounded by water molecules (solvated) and drift apart.  In a dilute aqueous solution of calcium chloride for example, solvated calcium ions and solvated chloride ions move independently of each other and are both evenly distributed throughout the solution.  This process can be represented by a chemical equation:

CaCl2 (s)    Ca2+ (aq)  +  2 Cl- (aq)

Notice the coefficient (2) before the symbol for chloride ion.  We do not write a subscript, Cl2, to balance the equation because the process does not produce chlorine molecules.  The equation is read as: One mole of solid calcium chloride dissolves in water to give one mole of solvated calcium ions and two moles of solvated chloride ions.  The symbol aq represents an unspecified number of water molecules.

Molarity

We have seen previously that the composition of a mixture can be described quantitatively using percentage by mass.  For example, a 4.2% solution of sodium chloride contains 4.2 g of sodium chloride per 100 g of solution.  Another common measure of composition for solutions is molarity, the number of moles of solute per liter of solution.  For example, a 1.5 M (read 1.5 molar) solution of sodium chloride has 1.5 moles of sodium chloride per liter of solution.

To calculate the molarity of a solution of known composition, we divide the amount of solute (in moles) by the volume of solution (in liters).  For example, if we prepare 50 mL of solution by dissolving 3.5 g of zinc chloride in water, the molarity is calculated as follows:

Molar mass of ZnCl2 = 65.39 + (2 x 35.45) = 136.29 g


Molarity is widely used in chemistry because it allows us to quickly calculate the amount of solute (in moles) associated with a measured volume of solution.  For example, how many moles of sodium chloride would be contained in 235 mL of 0.85 M solution?  To solve the problem in a way that allows us to cancel units, we need to remember that molar (M) means moles of solute per liter of solution, so the concentration gives us a conversion factor:

Here is another example.  What volume of 0.25 M calcium chloride solution would contain 4.0 x 10-2 mol of chloride ions?
 

Notice the mole ratio - every mole of calcium chloride provides two moles of chloride ions to the solution.

 

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