Chem 112 Saturday Spring 2000
Things to know for Exam 2

Chapter 10

Units Why? Humans have set units to all things measured. In some instances, the units are simple as in how humans measure time. The base unit is seconds, symbolized by "s". Units have been decided for distance, inches, feet, yards, miles, meters, kilometers, and for speed, miles per hour, kilometers per hour. Units are used in order to set up a communication standard for information transfer.

In the laboratory, chemists have decided that in addition to the commonly used mass, time, and volume units there was a need for determining and communicating amounts of substances. Most chemical reactions involve masses or volumes because our equipment is designed to measure mass and/or volume.

Most chemical reactions occur in gaseous or liquid phases. Both of these phases are readily measurable as volumes. Because all gases have similar densities, amount of a gas are often determined by measuring a volume at a set temperature and pressure. Liquids can also be measured by volumetric means. Solids however lend themselves only to mass measurement. Since solids seldom react in solid form, they are quite commonly dissolved in a solvent so that the reaction can be studied. Thus a mass per volume measurement provides an excellent means of measuring quantities of these materials.

Mass is measured in grams but chemical reactions occur in molar quantities. It is apparent that the use of mass per volume does not relate directly to a reaction. Moles and mass can be readily converted through the use of the molar mass of any substance. Chemist therefore decided to create a unit with is used known as molarity, M. However moles can't be measured directly so other units relating mass and volume have been developed. These mass/volume units are collectively known as concentrations.

There are many concentration units. For low concentrations, the units used are usually:

    ppm - parts per million which means the number of parts of one 
          substance in a million parts total. 


    ppb - parts per billion meaning the number of parts of one 
          substance in a billion total parts.

As concentrations increase, the common term used to express their values is molarity, M. Molarity is a measure directly related to reactions since it expresses moles of substance per liter of solution.

In some instances, the unit, molality, m, is used when the masses of the solute and the solvent have been measured. Molality is the mass of solute, that substance their in the least quantity, divided by the mass of the solvent in kilograms. The solvent is the substance in the greater amount.

Another unit used in the past was Normality, N. Normality worked when the reaction was known because it expressed the concentrations of "+" species which react with equivalent "-" species.

Sometimes simple concentration based upon percents are used. The common unit here is "mass percent". The calculations are simply but have no direct correlation to any particular reaction.

                       mass of solute
    mass percent = ---------------------- x 100
                   total mass of solution

Another concentration unit is mole fraction where the moles of one substance are divided by the total number of moles in the solution. The sum of the mole fractions of a single solution always add to 1 so mole fraction can be used as a quick check on the quality of measurement and calculation.

The concentration units that 112 students should be familiar with are molarity, M and ppm. Let's look at molarity:

Moles cannot be measured directly so the most common measurement used is mass. The molar mass is then used to convert the mass to moles. Here's an example.

In making molar solutions, it is recognized that solids and their ions even when dissolved occupy space. In order to manage this volume, the solid is put in the flask BEFORE adding the solvent. In solid form, more space is occupied than in dissolved form so the solid is first dissolved before all the water is added. Once the ions are in solution space (volume) changes no longer occur. A molal solution doesn't deal with the volume of the solid or the ions but merely uses the mass of the substance.

Here's a molar solution formulation.

A technician weighs out 47 g of NaCl and places it in a 500 mL volumetric flask. Some distilled water is added and solution swirled until the NaCl is dissolved. Distilled water is added until the solution in the flask fills the neck and the meniscus is in the proper position with the bottom of the meniscus on the volume line. What is the resulting concentration?

Step 1) determine the molar mass of NaCl.

    1 mole NaCl contains one mole Na; mass = 22.99 g
    1 mole NaCl contains one mole Cl; mass = 35.45 g
                                total mass:  58.44 g

By definition, a 1 M solution contains 1 mole of a substance in 1 L of total solution. So if the technician had placed 58.44 g of NaCl in a 1 L flask the concentration would have been 1 M.

In this case the flask is 500 mL which is 0.500 L and the mass is 47 g. Step 2) Determine how many moles of substance are being used. Divided the mass used by the molar mass like so:

        47 g NaCl
        ------------  = 0.804 mole NaCl
        58.44 g/mole 

If we put the 47 g in a 1 L flask the resulting solution would be, 0.804 M but we are placing the 47 g in half a liter. This is in effect the same as adding twice as much solid. So in order to determine the concentration made by placing 47 g in 500 mL, the 0.804 M value must be doubled. The answer is that 47 g NaCl in 500 mL produces a 1.608 M solution of NaCl.

For comparison let's look at the ppm reading this solution would give. We have:

                  47 g NaCl
This is the same as:

                  47 x 103 mg NaCl

A L of water is the same as 1000 mL of water. With a density of 1 g / mL this means 1 L water has a mass of 1000 g which is a kilogram.


                47 x 103 mg NaCl
                      1 kg

The student should recognize that mg/kg is equivalent to ppm since there are 1,000,000 mg in a kg. Thus our NaCl solution has 47000 ppm NaCl The ppm of each element in NaCl is not 47000 ppm but the solid has a concentration of 47000 ppm. To determine the ppm of Na or Cl, one must determine what fraction of the 47 g is Na and what part is Cl. This is calculated by multiplying the 47 g times the mole fraction of Na and of Cl in NaCl. The mole fractions are determined so:
        22.99                     35.45
        ----- = .393 for Na  and  ----- = .607 for Cl
        58.44                     58.44  

Using these fractions we see that of the 47 g, 47 * .393 = 18.47 g are Na and 47 * .607 = 28.53 g are Cl.

So the Na ppm is 18470 ppm and the Cl is 28530 ppm. Notice that because of conservation of mass these values add to 47000.

Hopefully, the reader can see that using ppm for large concentrations is not a good way of working. So at large concentrations molarity is used while at small concentrations ppm or even ppb are used. The ppm and ppb levels are controlled by the solubility of substances. The majority of minerals in nature are of low solubility resulting in low ppm or ppb levels of dissolved ions in solution.

Principles of Solubility
Solutes dissolve according to set principles of solubility. The rules of solubility were discussed in an earlier chapter. One key rule is that a polar solvent like water only dissolves substances which are polar. Another way of saying this is that like dissolves like; or oil and water don't mix.

In order for a substance to dissolve the solute-solvent attracts must overcome the solute-solute and solvent-solvent attractions. That is the attraction of the one substance for the other must be greater than the attraction of each substance for itself. In lab, the internal attractive forces were seen when surface tension was studied.

Water is a polar substance meaning it has a "zone" of positive charge and a zone of negative charge. These zones allow water to "cling" to itself with intermolecular (between molecules) attractive forces. Other polar substances can overcome this self-attraction and slip between the water molecules. Ions are the ultimate in polar materials since one of each pair is positive charged and the other is negative charged. Because ions can actually separate from each other in solution, their entire positive or negative charge can be used in intermolecular attractions.

Polar is is due to an imbalance of the attractions of electrons to various nuclei. Oxygen is very electronegative which means it it has a strong attraction for electrons. When an oxygen bonds to another atom, the electrons in the bond are held closer to the oxygen nucleus than to the nucleus of the other element. Since there are more electrons near the oxygen than there are protons in the nucleus to balance the negative electron charge, the oxygen exhibits a net negative polarity. The other element has more protons in its nucleus than there are electrons in nearby orbit so that atom has a net positive polarity. One common attribute to many non-ionic water soluble substance is an hydroxyl group, OH. The OH bond is polar and thus compounds which have OH groups can "slip" between the water molecules and solubilize (dissolve). Table sugar has 10 such groups and even though sugar is a large organic molecule it dissolves readily in water.

There are basically three types of solutions: undersaturated, saturated, and supersaturated.

Undersaturation means that less material has dissolved in the solution than can be dissolved under the conditions. For example, a single teaspoon of table salt in a glass of water. All the salt dissolves and no solid is left.

Saturation means that as much substance as is possible under the existing conditions is in solution. Technically only a single molecule or formula unit of substance need be left undissolved however a more reasonable determination is that a single crystal remains visible in the solution.

Supersaturation is a special condition affected most commonly by raising the temperature (making syrup or rock candy) or pressure (carbonated beverages). Quite often the temperature is raised so that a larger amount of substance can be dissolved than is normal. Le Chatelier's Principle causes these supersaturated solutions to try to return to proper equilibrium. Note that once a syrup solution is made only minor disturbances can cause the sugar to crystallize out. Crystallization will only stop when the sugar in solution drops to that concentration supported by at the saturated state.

Everyone recognizes how a CO2 supersaturated solution reacts. PfffT!!!!

Temperature can thus be seen to generally increase the solubility of solids. Gases on the other hand become less soluble as temperature increases. Gases move more rapidly at higher temperatures and can escape the intermolecular attractive forces.

At high pressures the solubility of a gas is high. At low pressures, the solubility is low. To make bubble-free ice, water can be first boiled so the gases escapes OR the water-filled trays can be placed in a vacuum chamber and the air "sucked" out.