Kaplan MCAT General Chemistry Review

In spite of the fact that ionic compounds are made of ions, solid ionic compounds tend to be poor conductors of electricity, because the charged particles are rigidly set in place by the lattice arrangement that serves as the basic framework for crystalline solids. In aqueous solutions, however, the lattice arrangement is disrupted by the ion–dipole interactions between the ionic constituents and the water molecules. The freed-up ions are now able to move around, and as a result, the solution of ions is able to conduct electricity. Solutes that enable their solution to carry currents are called electrolytes. The electrical conductivity of aqueous solutions is governed, then, by the presence and concentration of ions in the solution. Pure water, which has no ions other than the very few hydrogen ions and hydroxide ions that result from water’s low-level autodissociation, is a very poor conductor.

Bridge

Because electrolytes ionize in solution, they will produce a larger effect on colligative properties (see Chapter 8) than one would expect from the given concentration.

The tendency of an ionic solute to dissociate into its constituent ions in water may be high or low. A solute is considered a strong electrolyte if it dissociates completely into its constituent ions. Examples of strong electrolytes include certain ionic compounds, such as NaCl and KI, and molecular compounds with highly polar covalent bonds that dissociate into ions when dissolved, such as HCl in water. A weak electrolyte, on the other hand, ionizes or hydrolyzes incompletely in aqueous solution, and only some of the solute is dissolved into its ion constituents. Examples include Hg₂I₂ (K_sp = 4.5 × 10^-29), acetic acid and other weak acids, ammonia, and other weak bases (see Chapter 10, Acids and Bases). Many compounds do not ionize at all in aqueous solution, retaining their molecular structure in solution, which usually limits their solubility. These compounds are called nonelectrolytes and include many nonpolar gases and organic compounds, such as O₂ (g), CO₂ (g), and glucose.

Concentration

Concentration denotes the amount of solute dissolved in a solvent. There are many different ways of expressing concentration, and different units have been standardized that you may encounter in everyday situations. For example, alcohol content in liquors like vodka, gin, or rum is expressed in volume percent (volume of solute divided by volume of solution times 100 percent). Alcoholic proof is twice the volume percent. The sugar content of orange juice and other fruit juices is measured in units of degrees Brix (°Bx), which is a weight (actually mass) percent: mass of glucose divided by mass of solution times 100 percent.

UNITS OF CONCENTRATION

On the MCAT test, you will work with concentrations of solutions commonly expressed as percent composition by mass, mole fraction, molarity, molality, and normality.

Percent Composition by Mass

The percent composition by mass (w/w%) of a solution is the mass of the solute divided by the mass of the solution (solute plus solvent), multiplied by 100 percent.

MCAT Expertise

It is important to have a good idea of how to work with all of these ways of expressing concentration because more than one may show up on Test Day.

Example: What is the percent composition by mass of a salt water solution if 100 g of the solution contains 20 g of NaCl?

Solution:× 100 + 20% NaCl solution

Mole Fraction

The mole fraction (X) of a compound is equal to the number of moles of the compound divided by the total number of moles of all species with the system. The sum of the mole fractions in a system will always equal 1. Mole fraction is used to calculate the vapor pressure depression of a solution, as well as the partial pressures of gases in a system.

Example: If 92 g of glycerol is mixed with 90 g of water, what will be the mole fractions of the two components? (MW of H₂O = 18; MW of C₃H₈O₃ = 92.)

Solution:

MCAT Expertise

Note that for dilute solutions, the volume of the solution is approximately equal to the volume of solvent used, which simplifies our need for calculation on Test Day.

Molarity

The molarity (M) of a solution is the number of moles of a solute per liter of solution. Solution concentrations are usually expressed in terms of molarity, and you will be working mostly with molarity on the MCAT. The molarity of a solution is written using brackets. Please note that the volume term in the denominator of molarity refers to the solution volume, not the solvent volume used to prepare the solution—although often the two values are close enough that we can approximate the solution volume by the solvent volume. We use molarity for the law of mass action, rate laws, osmotic pressure, pH and pOH, and the Nernst equation.