4. "Concentration and amount mean the same thing."
Some students have difficulty distinguishing the concepts of amount and concentration. By international convention (SI), the amount of a substance is measured in moles. Solution concentration is frequently expressed as Molarity (M), which is the ratio of moles of solute to liters of solution. Thus, simply stating Molarity does not give amount of substance; the volume must also be known. The amount of acid, in moles, is found from the relationship;

Moles = Molarity X Volume(in liters)

Thus, the amount of solute in these solutions:

1.0 L of 2.0 M
2.0 L of 1.0 M
0.50 L of 4.0 M

is 2.0 mol in each. It is apparent that the concentrations, 2.0, 1.0 and 4.0 M are not identical. Concentration is not, then, the same as amount. This is emphatically illustrated by discussing the relative effects of 1 drop of concentrated sulfuric acid(0.05 ml of 18M H2SO4) and 9 L of 0.1M sulfuric acid. While both contain 0.9 moles of acid molecules, a single drop of the former solution causes a painful burn, while the latter is harmless. Demonstrate this using a paper towel rather than skin! The concentrated acid eats through the paper, leaving a carbonized patch; but dropping the towel into the plastic bucket of the diluted acid(1 drop in 9L) causes no change.

You may illustrate this with food coloring (see SourceView: Problem Solving, also). Place 3-4 drops of food coloring in a 100-250 mL graduated cylinder filled about a third full with water. Stir. Set this on an illuminated overhead projector stage. Add water, having students observe from top and side. Ask if any more dye was added. Point out that the side view shows the dilution effect while the top view (color constant) indicates the total amount of dye substance.

The color varies according to Beer's Law.

A = abc where,

A = Absorption
a = molar absorptivity (a constant for a given colored species)
b = light path length or solution "thickness", and
c = concentration of colored species

As water was added, a remained constant, c decreased, cross section b remained constant. Thus from the side, A = abc decreased. From above, however, b increased as much as c decreased so A = abc was constant.

5. "Ammonium hydroxide is the name of the solution formed when ammonia gas dissolves in water."
Ammonia gas does readily dissolve in water. After it has dissolved, the equilibrium represented by the following expression is eventually attained.

Sometimes this solution is called ammonium hydroxide. However, the equilibrium lies far to the left; few ammonium ions, NH4+, or hydroxide ions, OH-, are present. The compound NH4OH has not been isolated. Water solutions of ammonia are weak electrolytes. Aqueous ammonia is a better name. Ammonia is a weak base.

Acids and Bases
(Page 26)