Kaplan MCAT General Chemistry Review

MCAT Expertise

An experimental-type passage that involves a chemical reaction may include a pseudo-discrete question that involves finding the percent yield.

Example: What is the percent yield for a reaction in which 27 g of Cu is produced by reacting 32.5 g of Zn in excess CuSO4 solution?

Solution: The balanced equation is as follows:

Zn (s) + CuSO₄ (aq) Cu (s) + ZnSO₄ (aq)

Calculate the theoretical yield for Cu.

Finally, determine the percent yield.

MCAT Expertise

When we are given an excess of one reagent on the MCAT, we know that the other reactant is the limiting reagent. Be sure to take advantage of these easy cues when they appear on Test Day!

Conclusion

We began our consideration of compounds with a particularly odoriferous one: benzaldehyde. As a compound, it is made from constituent atoms of different elements in a set ratio defined by its empirical or molecular formula. Each molecule of a compound has a defined mass that is measured as its molecular weight. The mass of one mole of any compound is determined from its molar mass in the units of grams/mole. We reviewed the basic classifications of reactions commonly tested on the MCAT: combination, decomposition, single-displacement, and double-displacement reactions. Furthermore, we are now confident in our understanding of the steps necessary to balance any chemical reaction; we are ready to tackle more stoichiometric problems in preparation for Test Day.

Before moving to the next chapters discussing chemical kinetics and thermodynamics, let us offer our heartiest congratulations to you. If you have been reading these chapters in order, you have now completed one-third of this general chemistry review! Take note of this, in part, because it is an important milestone in your progress toward success on Test Day and you should be proud of your accomplishments, but mostly because these first four chapters have introduced you to the fundamental concepts of chemistry—everything from the structure of the atom and trends of the elements to bonding and the formation of compounds. The understanding you have gained so far will be the foundation for your comprehension of even the most difficult general chemistry concept tested on the MCAT. Keep moving forward with your review of general chemistry; don’t get stuck in the details. Those will come to you best through the application of the basic principles to MCAT practice passages and questions.

And remember, now that you’ve read this chapter, the next time somebody says, “Oh, what stinks?” your response can be more than just, “Oh, sorry.”

CONCEPTS TO REMEMBER

A compound is a pure substance composed of two or more elements in a fixed proportion. Compounds can react with other elements or compounds to form new compounds and be broken down by chemical means to produce their constituent elements or other compounds, which can themselves go on to become involved in other reactions.

Molecular weight is the mass in amu of the constituent atoms in a compound, given by the molecular formula, which gives the exact number of atoms of each element in a compound. Empirical formula weight is the mass of the constituent atoms in a compound’s empirical formula, which is the smallest whole number ratio of the elements in a compound. Molar mass is the mass in grams of one mole (6.022 × 10²³ molecules) of a compound.

An equivalent is a measure of capacity to react in a certain way. One equivalent is an amount of a chemical compound equal to one mole of hydrogen ions or hydroxide ions in acid-base reactions or one mole of electrons in redox chemistry. Gram equivalent weight is the mass in grams of a compound that will yield one equivalent of hydrogen ions, hydroxide ions, or electrons. Normality is the ratio of equivalents per liter.

Combination reactions occur when two or more reactants combine to form one product.

Decomposition reactions occur when one reactant is chemically broken down into two or more products, usually by heat or electrolysis.

Single-displacement reactions occur when an atom or ion of one compound is replaced by an atom or ion of another element.

Double-displacement reactions occur when elements from two different compounds trade places with each other to form two new compounds. Neutralization reactions are a specific type of double displacement in which an acid reacts with a base to produce a solution of salt and water.

Net ionic equations ignore spectator ions to focus only on the species that actually participate in the reaction.

The steps for balancing chemical equations are as follows:

—Balance the nonhydrogen and nonoxygen atoms.

—Balance the oxygens.

—Balance the hydrogens.

—Balance charge if necessary.

Balanced reactions are essential for calculating limiting reactant (the reactant that will be consumed first in a chemical reaction) and yields. Theoretical yield is the maximum amount of product that can be formed, assuming all limiting reactant is consumed. Actual yield is the amount of product collected from a chemical reaction. Percent yield is the ratio of actual yield divided by theoretical yield, multiplied by 100 percent.

EQUATIONS TO REMEMBER

Practice Questions

1. Which of the following best describes ionic compounds?

A. Ionic compounds are formed from molecules containing two or more atoms.

B. Ionic compounds are formed of charged particles and are measured by molecular weight.

C. Ionic compounds are formed of charged particles that share electrons equally.

D. Ionic compounds are three-dimensional arrays of charged particles.

2. Which of the following has a formula weight between 74 and 75 grams per mole?

A. KCl

B. C₄H₁₀O

C. [LiCl]₂

D. BF₃

3. Which of the following is the gram equivalent weight of H₂SO₄?

A. 98.08 g/mol

B. 49.04 g/mol

C. 196.2 g/mol

D. 147.1 g/mol

4. Which of the following molecules CANNOT be expressed by the empirical formula CH?

A. Benzene

B. Ethyne

C.

D.

5. In which of the following compounds is the percent composition of carbon closest to 63 percent?

A. Acetone

B. Ethanol

C. C₃H₈

D. Methanol

6. What is the most accurate characterization of the reaction shown?

Ca(OH)₂ (aq) + H₂SO₄ (aq) CaSO₄ (aq) + H₂O ()

A. Single-displacement

B. Neutralization

C. Double-displacement

D. Redox

7. In the reaction shown, if 39.03 g of Na₂S are reacted with 113.3 g of AgNO₃, how much, if any, of either reagent will be left over once the reaction has gone to completion?