194 ❯ STEP 4. Review the Knowledge You Need to Score High
❯ Answers and Explanations
Question 1
(a) Solution 4, because the ammonium ion is a weak acid.
You get 1 point for picking solution 4 and 1 point for saying the ammonium ion
(NH 4 +) is a weak acid or that it undergoes hydrolysis.
(b) Solution 3, because the solute is a nonelectrolyte.
Give yourself 1 point for picking solution 3 and 1 point for saying it is a nonelec-
trolyte or that it does not ionize.
Total your points. There are 4 points possible.
Question 2
(a) The ammonium ion, from the ammonium chromate, will not form a precipitate since
most ammonium compounds are water soluble. Therefore, the precipitate must con-
tain the chromate ion combined with a cation from one of the solutions. Solution (2) is
a nonelectrolyte; therefore, there are no cations present to combine with the chromate
ion. The potassium and sodium ions, from solutions (1) and (3), give soluble salts like
the ammonium ion. This leaves only solution (5), barium chloride, which will give a
precipitate. The formula of the precipitate is BaCrO 4.
You get 1 point for picking the correct solution.
(b) Solution (3), sodium carbonate, is the most basic. Since the carbonate ion is the conju-
gate base of a weak acid, it will undergo significant hydrolysis to produce a basic solution.
You get 1 point for picking the correct solution and giving the correct explanation.
(c) Methyl alcohol is a nonelectrolyte, so its solutions do not conduct electricity. The
remaining solutions contain ionic salts, which, in general, are electrolytes in solution.
You get 1 point for picking the correct solution and giving the correct explanation.
(d) Solution (4), ammonium chromate, is yellow. Most solutions containing a transition
metal ion are colored.
You get 1 point for picking the correct solution.
Total your points. There are 4 points possible.❯ Rapid Review
● A solution is a homogeneous mixture composed of a solvent and one or more solutes. A
solute is a substance that dissolves in the solvent and is normally present in smaller amount.
● The general rule of solubility is “like dissolves like.” This means that polar solvents dissolve
polar solutes and nonpolar solvents dissolve nonpolar solutes. Remember, however, that
simply quoting this rule will not be sufficient as an explanation in the free-response section.
● A saturated solution is one in which the maximum amount of solute is dissolved for a
given amount of solvent at a given temperature. Any solution with less than the maxi-
mum solute is called unsaturated. A solution with greater than maximum solute is super-
saturated (an unstable state).
● For the chemist the most useful unit of concentration is molarity (M), which is the
moles of solute per liter of solution. Know how to work molarity problems. Be careful
not to confuse molarity, M or [ ], with moles, n or mol.
● Electrolytes conduct an electrical current when melted or dissolved in a solvent, whereas
nonelectrolytes do not.
● A colloid is a mixture in which the solute particle size is intermediate between a true
solution and a suspension. If a light is shone through a colloid, the light beam is visible.
This is the Tyndall effect.