Stoichiometry is the term that applies to the quantitative aspects of chemical composition and of chemical reactions.
The chemical NaCl contains 1 mole of Na+ ions and 1 mole of Cl- ions in 1 mole of NaCl. Since 1 mole of Na atoms has a mass of 22.990 g and 1 mole of Cl atoms has a mass of 35.453 g, 1 mole of NaCl has a mass of 22.990 + 35.453 = 58.443 g. The fraction of Na in NaCl is 22.990 / 58.443 = 0.3934 (39.34% by weight).
The chemical reaction of 2H2 + O2
![Rightwards Arrow](https://biology-forums.com/Smileys/default/27a1.png)
2H2O states that 2 moles of H2 react with 1 mole of O2 to yield 2 moles of H2O. A mole of H atoms has a mass of 1.0079 g, so a mole of H2 has a mass of 2x1.0079 = 2.0158 g and 2 moles of H2 has a mass of 2x2.0158 = 4.0316 g. A mole of O atoms has a mass of 15.9994 g so a mole of O2 has a mass of 31.9988 g. The mass of 2 moles of H2O is 2 x (2x1.0079 + 15.9994) = 36.0124 g.
The quantitative aspects of the chemical reaction are-
4.0316 g H2 react with 31.9988 g of O2 to yield 36.0124 g of H2O
The only hard part of stoichiometry is remembering how to spell it!