What is Stoichiometry?

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What is a mole ratio?

The Meaning of the Word

The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry, so be prepared for lots of calculator use.

Jeremias Benjaim Richter (1762-1807) was the first to lay down the principles of stoichiometry. In 1792 he wrote:

"Die stöchyometrie (Stöchyometria) ist die Wissenschaft die quantitativen oder Massenverhältnisse zu messen, in welchen die chymischen Elemente gegen einander stehen."
[Stoichiometry is the science of measuring the quantitative proportions or mass ratios in which chemical elements stand to one another.]

What You Should Expect

The most common stoichiometric problem will present you with a certain amount of a reactant and then ask how much of a product can be formed. Here is a generic chemical equation:

2 A + 2B ---> 3C

Here is a typically-worded problem (often called a "mass-mass" problem):

Given 20.0 grams of A and sufficient B, how many grams of C can be produced?

You will need to use molar ratios, molar masses, balancing and interpreting equations, and conversions between grams and moles. If you struggled with those in class, welcome to the club. Go back and review them if you need to, because if you can't do that stuff, you can't do stoichiometry.

The Steps Involved in Solving Mass-Mass Stoichiometry Problems

  1. Make sure the chemical equation is correctly balanced.
  2. Using the molar mass of the given substance, convert the mass given in the problem to moles.
  3. Construct a molar proportion (two molar ratios set equal to each other) following the guidelines which will be set out in the tutorial. Use it to convert to moles of the unknown.
  4. Using the molar mass of the unkown substance, convert the moles just calculated to mass.

The ideas involved will be gone over in detail in other tutorials. The topic usually presented first is the concept of a mole ratio.

What is a mole ratio?

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