Experiment 4-2

Measuring Concentration

Background Information

We know that some materials seem to disappear when dissolved in water.  From experiment 2-1 we know that the mass does not change during this process, so apparently the dissolving material does not vanish, but becomes an often invisible part of the solution.  In the previous experiment with a colored solute we noted the concentration seemed to vary depending on BOTH the amount of solute and the amount of solvent.

The uniform color of each particular solution in the previous experiment suggests that concentration is a characteristic property of a solution.  But since color varied from one solution to another, the concentration of solutions can vary.  Thus it will be useful to develop at least one method for determining and communicating concentration.  This skill is of great practical and financial benefit in our society.

The color of solutions #1 and #3 in Experiment 4.1 were about the same although #3 has twice as much solute AND twice as much solvent.  This suggests that amount of solute for EQUAL quantities of solvent might provide a useful definition of concentration.  On first thought, it might seem that the mass of solute per cubic centimeter of volume of solvent might be an ideal way of measuring and communicating concentration.  However g/cm3 has already been developed (see Experiment 3-3) for measuring density.  Therefore it is customary to use a different volume for measuring and communicating concentration.

We shall define concentration as:
mass of solute per 100 cm3 of solvent;  the units will be g/100cm3

Experiment

Purpose:  In this experiment we shall attempt to measure the concentration of a saturated solution of table salt, a substance with chemical formula of NaCl (sodium chloride).  Chemists prefer to label simple materials by the elements within.  To save time they use a shorthand developed by the Swedish chemist, Berzelius.

Materials needed:
• table salt, (NaCl)
• water
• balance
• container for mixing and second container to be used for evaporating solution
• heating device for evaporating solution

Procedure:

1. Measure a reasonable amount of water (enough to be accurately measured, but little enough to later evaporate), perhaps 20cm3.
2. Stir in enough salt (perhaps a 10mL or a table spoon) to saturate the solution.  Excess salt must remain undissolved.
3. Carefully weigh the empty container to be used for evaporating.
4. Carefully decant (pour off the liquid leaving all solid behind) the liquid into the weighed container.
5. Evaporate the water in the solution until dry.  CAUTIONS:  Avoid heating a glass container on its bottom;  it will often shatter.  ALSO the container may get very hot after all water has evaporated.
6. After cooling, reweigh to determine the mass of salt that had been dissolved.
7. Knowing the mass of dissolved salt and the volume of solvent, calculate the concentration in g/100cm3.

Record your results in your science journal.  Write a Formal Report if you need to earn credit.

Discussion of Calculations and Conclusion:  The concentration of a saturated solution is known as the solubility of the material.  There are a number of additional methods for measuring and reporting concentration.  These typically are learned in Chemistry.