Kaplan MCAT General Chemistry Review

Sometimes chemistry lab professors like to torture students by grading them on the purity of their products and the yields of their experiments. The enjoyment for the professor comes from watching crazed premed students desperately scraping their glassware in the forlorn hope that they might be able to capture just enough errant product to increase yield by a few measly percentage points. The “yield” of a reaction is either the amount of product predicted (theoretical yield) or obtained (raw or actual yield) when the reaction is carried out. Theoretical yield is the maximum amount of product that can be generated, predicted from the balanced equation, assuming that all of the limiting reactant is consumed, no side reactions have occurred, and the entire product has been collected. Theoretical yield, as your experience has most certainly taught you by now, is rarely ever attained through the actual chemical reaction. Actual yield is the amount of product that you are actually able to obtain. The ratio of the actual yield to the theoretical yield, multiplied by 100 percent, gives you the percent yield, and this number is the fragile thread by which so many premeds fear their future careers hang.

Percent yield = (Actual yield/Theoretical yield) × 100%

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.