Kaplan MCAT General Chemistry Review

Next, find the simplest whole number ratio of the elements by dividing the number of moles by the smallest number obtained in the previous step.

Finally, the empirical formula is obtained by converting the numbers obtained into whole numbers (multiplying them by an integer value).

C₁H_1.33O₁ × 3 = C₃H₄O₃

C₃H₄O₃ is the empirical formula. To determine the molecular formula, divide the molar mass by the formula weight. The resultant value is the number of empirical formula units in the molecular formula.

The empirical formula weight of C₃H₄O₃ is

Method Two: When the molar mass is given, it is generally easier to find the molecular formula first. This is accomplished by multiplying the molar mass by the given percentages to find the grams of each element present in one mole of compound, then dividing by the respective atomic weights to find the mole ratio of the elements:

Thus, the molecular formula, C₉H₁₂O₉, is the direct result.

The empirical formula can now be found by reducing the subscript ratio to the simplest integral values.

Key Concept

The molecular formula is either the same as the empirical formula or a multiple of it. To calculate the molecular formula, you need to know the mole ratio (this will give you the empirical formula) and the molecular weight (molecular weight ÷ empirical formula weight will give you the multiplier for the empirical formula to molecular formula conversion).

Types of Chemical Reactions

This section reviews the major classes of chemical reactions. As it has probably already become apparent to you in your inorganic and organic chemistry classes, it would be quite impossible to memorize every single individual reaction that could occur. Fortunately, there is no need to memorize any reaction, as long as you take the time now and throughout your preparation for the MCAT to learn and understand the recognizable patterns of reactivities between compounds. Some classes of compounds react in very “stereotyped” ways, or at least appear to react in stereotyped ways, because of the MCAT’s focus on a particular subset of a chemical’s reactivities. The MCAT has a tendency to typecast certain compounds, in spite of repeated protests made by the compounds’ talent agents.

COMBINATION REACTIONS

A combination reaction has two or more reactants forming one product. The formation of sulfur dioxide by burning sulfur in air is an example of a combination reaction.

S (s) + O₂ (g) SO₂ (g)

Key Concept

Combination reactions generally have more reactants than products.

A + B C

DECOMPOSITION REACTIONS

A decomposition reaction is the opposite of a combination reaction: A single compound reactant breaks down into two or more products, usually as a result of heating or electrolysis. An example of decomposition is the breakdown of mercury (II) oxide. (The [delta] sign over a reaction arrow represents the addition of heat.)

SINGLE-DISPLACEMENT REACTIONS

Sometimes one atom in a molecule gets tired of the other (“I love you, but I’m not in love with you…”), and divorce becomes inevitable when something better comes along. This is a single-displacement reaction: an atom (or ion) of one compound is replaced by an atom of another element. For example, zinc metal will displace copper ions in a copper sulfate solution to form zinc sulfate.

Zn (s) + CuSO₄ (aq) Cu (s) + ZnSO₄

Single-displacement reactions are often further classified as redox reactions, which will be discussed in great detail in Chapter 11, Redox Reactions and Electrochemistry. Not to carry the analogy to the point of ridiculousness, but just to make the point: Cu in CuSO₄ has an oxidation state of +2, but when it leaves the compound, it gains two electrons (the Cu⁺² is reduced to Cu)—you may think of this as Cu⁺² getting an alimony settlement or gaining back its dignity for leaving that no-good cheater, SO₄^2-. On the other side, Zn loses its dignity (in the form of two electrons) when it desperately throws itself into the arms of SO₄^2-.

DOUBLE-DISPLACEMENT REACTIONS

Well, if you think atoms are acting scandalously in single-displacement reactions, just wait until you get a load of the atomic depravity in double-displacement reactions. In double-displacement reactions, also called metathesis, elements from two different compounds swap places with each other (hence, the name double-displacement) to form two new compounds. This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas or when two of the original species combine to form a weak electrolyte that remains undissociated in solution. For example, when solutions of calcium chloride and silver nitrate are combined, insoluble silver chloride forms in a solution of calcium nitrate.