Stoichiometry
There are many different classifications of matter in chemistry, as we cannot call two completely different chemicals the same. Firstly, it is called a mixture if we can physically separate it. Then, if the mixture has a uniform composition, it is a homogenous mixture or a solution, but if not, it is a heterogenous mixture. Some examples of a homogenous mixture would be a solution or an alloy, and a heterogenous mixture would be sand or dirt.
If the substance cannot be physically separated, it is called a pure substance. Pure substances are split into compounds and elements based on whether they can be chemically separated. Compounds can change the properties of each element they are composed of. Sodium, a soft and extremely reactive metal, combines with chloride, a toxic gas, to create NaCl, commonly known as table salt.
Stoichiometry is the relationship between relative quantities of reactants and products in chemical reactions. The reactants are what is reacting (the left side of the reaction) and the products are what are being produced (the right side).
The key to stoichiometry is moles. A mole is a unit of measurement that can easily be compared between two pure substances on either side of a reaction. One mole of a substance can come from four different methods of calculation. The first, and simplest, is to use a mole comparison. If the reaction is:
2H + O -> H₂O
then the mole comparison of hydrogen, H, to oxygen, O, will be 2 to 1, as those are the coefficients in front of each comparative substance.
You can also use the molar mass of a substance. Let’s say that we have 2.016 grams of hydrogen gas. Since the molar mass of hydrogen is 1.008 grams/mole, we can divide 2.016 g by 1.008 g/mol and calculate our two moles of hydrogen. Every element has a different molar mass, which all get added up when there are multiple moles and elements in a compound. For example, NaCl has a molar mass of 58.44 g/mol which comes from sodium’s molar mass of 22.99 g/mol and chloride’s molar mass of 35.45 g/mol.
Using the number of particles of a substance, you can calculate the moles of that substance by multiplying the number of particles by Avogadro’s number, which is 6.023 x 10²³. That means if you have 1 mole of water, there are around 600 septillion particles in that 18.02 grams.
Finally, you can calculate moles using the volume of a gaseous substance at STP (temp of 273 K, 0° C, or 32° F, and pressure of 1 atm). One mole of a substance always occupies 22.4 liters at STP.
The formula of a compound is known as its molecular formula. It shows the amount of moles of each given element in the compound. The empirical formula is like the least common denominator of that formula, where it simplifies the number of moles down to its lowest possible number whilst keeping the ratio the same. Dinitrogen tetroxide, or N₂O₄, has an empirical formula of NO₂.
The empirical formula is useful when trying to identify substances. If you are told that a certain hydrocarbon CₓHᵧ has 12.011 g C and 2.016 g H, you can use stoichiometry to determine that there is one mole of carbon and two moles of hydrogen, giving you the empirical formula of CH₂, or methylene. However, this does not tell you what substance you have, as you could have ethene, C₂H₄.
Some elements are known as diatomic. These elements exist in their most common form as a compound with two moles of themselves. Dihydrogen, oxygen, dinitrogen, chlorine, bromine, iodine, and fluorine are all diatomic compounds of hydrogen, oxide, nitrogen, chloride, bromide, iodide, and fluoride.