Stoichiometry is a toolbox for describing chemical change that employs the mass quantity relationships between reactants and products.

In the earlier chapters of Atomic Theory, Periodic Properties, Chemical Bonding, and Intermolecular Forces, we discussed the structure of matter and some of the changes in structure that can occur at the atomic, molecular, or intermolecular level. It is important to remember, however, that Chemistry is a laboratory practice, and the scale of the laboratory is much larger than atoms and molecules. In order to describe the measurable relationships governing chemical change, a system of accounting is necessary which describes the products and reagents in chemical equations in measurable terms like mass and moles. Stoichiometry describes the body of accounting techniques for the purpose of describing chemical reactions at the laboratory scale.

It is the right stage of the course to take the time to cover Stoichiometry. Already this week in the Main Sequence, we have learned how to describe the most basic kind of physical thermodynamic system, an Ideal Gas within its surroundiongs, and now we need to equip ourselves with Stoichiometry to begin building the bridge to systems composed of real substances and the chemical reactions such systems may undergo.

Stoichiometric problem solving is a major focus during the first semester of General Chemistry, and many students get the impression of chemistry as an endless variation on plugging and chugging grams and moles, and their conceptual understanding of chemistry can suffer. Although this type of problem solving is relatively minor on the MCAT (MCAT physical science questions are much more likely to involve conceptual reasoning than number crunching), there will be enough stoichiometric terminology to prevent from achieving a superior score if they don't understand it. You definitely need to understand stoichiometric nomenclature and the basics of stoichiometric problem solving for the MCAT.