Kaplan MCAT General Chemistry Review

Ionic compounds do not form true molecules because of the way in which the oppositely charged ions arrange themselves in the solid state. As solids, they can be considered as nearly infinite, three-dimensional arrays of the charged particles that comprise the compound. Remember, in Chapter 3 we mentioned that NaCl in the solid state is a 6:6 coordinated lattice in which each of the Na⁺ ions is surrounded by six Cl^- ions and each of the Cl^- ions is surrounded by six Na⁺ ions. As you might imagine, this makes it rather difficult to clearly define a sodium chloride molecular unit. Because no molecule actually exists, molecular weight becomes meaningless, and the term formula weight is used instead. (However, this is a technical distinction over which you ought not to sacrifice too much sleep.)

Bridge

Ionic compounds form from combinations of elements with large electronegativity differences (that sit far apart on the periodic table), such as sodium and chlorine. Molecular compounds form from the combination of elements of similar electronegativity (that sit close to each other on the periodic table), such as carbon with oxygen.

MOLECULAR WEIGHT

We’ve mentioned already that the term atomic weight is a misnomer, because it is a measurement of mass, not weight (another distinction not worth any sacrifice of sleep), and the same applies here to our discussion of molecular weight: It’s really a measurement of mass. Molecular weight, then, is simply the sum of the atomic weights of all the atoms in a molecule, and its units are atomic mass units (amu). Similarly, the formula weight of an ionic compound is found by adding up the atomic weights of the constituent ions according to its empirical formula (see the following example), and its units are grams.

Example: What is the molecular weight of SOCl₂?

Solution: To find the molecular weight of SOCl₂, add together the atomic weights of each of the atoms.

MOLE

We defined the term mole in Chapter 1, Atomic Structure, but let’s briefly review it. A mole is a quantity of any thing (molecules, atoms, dollar bills, chairs, etc.) equal to the number of particles that are found in 12 grams of carbon-12. That seems like an awfully strange point of comparison, so all you really need to remember is that “mole” is a quick way of indicating that we have an amount of particles equal to Avogadro’s number, 6.022 × 10²³. One mole of a compound has a mass in grams equal to the molecular weight of the compound expressed in amu and contains 6.022 × 10²³ molecules of that compound. For example, 62 grams of H₂CO₃ (carbonic acid) represents one mole of the acid compound and contains 6.022 × 10²³ molecules of H₂CO₃. The mass of one mole of a compound, called its molar weight or molar mass, is usually expressed as g/mol. Therefore, the molar mass of H₂CO₃ is 62 g/mol. You may be accustomed to using the term molecular weight to imply molar mass. Technically this is not correct, but nobody is going to rap your knuckles with a ruler for this minor infraction.

The formula for determining the number of moles of a substance present is

mol = Weight of sample (g)/ Molar weight (g/mol)

Real World

Here we mention Avogadro’s number again and can see that the mole is just a unit of convenience, like the dozen is a convenient unit for eggs.

Example: How many moles are in 9.52 g of MgCl₂?

Solution: First, find the molar mass of MgCl₂.

1(24.31 g/mol) + 2(35.45 g/mol) = 95.21 g/mol

Now, solve for the number of moles.

EQUIVALENT WEIGHT

Equivalent weight and the related concept of equivalents are a source of confusion for many students. Part of the problem may be the context in which equivalents and equivalent weights are discussed: acid-base reactions, redox reactions, and precipitation reactions, all three of which themselves can be sources of much student confusion and anxiety. So let’s start with some very basic discussion, and then, in later chapters, we will see how these concepts and calculations apply to these three types of reactions.

Bridge

The idea of equivalents is similar to the concept of normality, which we will see when we study Acids and Bases in Chapter 10.