5 Steps to a 5 AP Chemistry 2019

184 ❯ STEP 4. Review the Knowledge You Need to Score High

Molality

Sometimes the varying volumes of a solution’s liquid component(s) due to changes in

temperature present a problem. Many times volumes are not additive, but mass is additive.

The chemist then resorts to defining concentration in terms of the molality. Molality (m)

is defined as the moles of solute per kilogram of solvent.

m = moles solute/kilograms solvent

Notice that this equation uses kilograms of solvent, not solution. The other concen-

tration units use mass or volume of the entire solution. Molal solutions use only the mass

of the solvent. For dilute aqueous solutions, the molarity and the molality will be close to

the same numerical value.

For example, ethylene glycol (C 2 H 6 O 2 ) is used in antifreeze. Determine the molality

of ethylene glycol in a solution prepared by adding 62.1 g of ethylene glycol to 100.0 g

of water.

molality=××=m

62.1 gCHO

100.0gHO

1,000 g

1kg

1mol CHO

62.1 gCHO

(^262) 10.0 CHO
2
262
262
262
The most common error is to use the total grams in the denominator instead of just the
grams of solvent.
Electrolytes and Nonelectrolytes
An electrolyte is a substance that, when dissolved in a solvent or melted, conducts an elec-
trical current. A nonelectrolyte does not conduct a current when dissolved. The conduc-
tion of the electrical current is usually determined using a light bulb connected to a power
source and two electrodes. The electrodes are placed in the aqueous solution or melt, and
if a conducting medium is present, such as ions, the light bulb will light, indicating the
substance is an electrolyte.
The ions that conduct the electrical current can result from a couple of sources. They
may result from the dissociation of an ionically bonded substance (a salt). If sodium chloride
(NaCl) is dissolved in water, it dissociates into the sodium cation (Na+) and the chloride
anion (Cl-). But, certain covalently bonded substances may also produce ions if dissolved
in water, a process called ionization. For example, acids, both inorganic and organic, will
produce ions when dissolved in water. Some acids, such as hydrochloric acid (HCl), will
essentially completely ionize. Others, such as acetic acid (CH 3 COOH), will only partially
ionize. They establish an equilibrium with the ions and the unionized species (see Chapter
15 for more on chemical equilibrium).



HCl(aq)H(aq) Cl (aq) 100%ionization

CH COOH(aq) H(aq)CHCOO(aq) partialionization

+

+

33

→+

+

−

−

Species such as HCl that completely ionize in water are called strong electrolytes, and

those that only partially ionize are called weak electrolytes. Most soluble salts also fall into

the strong electrolyte category.

Colligative Properties

Some of the properties of solutions depend on the chemical and physical nature of the

individual solute. The blue color of a copper(II) sulfate solution and the sweetness of a

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