The gram-per-mole conversion factor in stoichiometry is almost always present, and it allows chemists to predict what weights of materials are needed for a chemical reaction. For example, if hydrochloric acid reacts with the base sodium hydroxide to produce table salt and water, stoichiometry calculations can predict how much acid and how much base is needed so neither is left over and only salt and water remain in the solution that is produced. The calculations start with moles of each substance, and the conversion factors change the moles to weight.
TL;DR (Too Long; Didn't Read)
Stoichiometry lets chemists use the grams-per-mole conversion factor to calculate how much of each reactant is required in a chemical reaction. According to the Law of Conservation of Mass, chemical reactions are balanced, with the same number of atoms of each element going into a reaction as are found in the reaction products. The grams-per-mole conversion factor can be used to predict how much of each material is needed so none is left over, and how much of each reaction product will result from the reaction.
The Law of Conservation of Mass
According to the Law of Conservation of Mass, first proposed by the French 18th-century chemist Antoine Lavoisier, mass is neither created nor destroyed in a chemical reaction. This means that the number of atoms of each element going into a chemical reaction is always the same as the atoms in the reaction products. As a result, chemical reactions are balanced, with equal numbers of atoms on each side, even though they may be combined differently to form different compounds.
For example, when sulfuric acid, H2SO4, reacts with sodium hydroxide, NaOH, the unbalanced chemical equation is H2SO4 + NaOH = Na2SO4 + H2O, producing sodium sulfate and water. There are three hydrogen atoms on the left side of the equation but only two on the right side. There are equal numbers of sulfur and oxygen atoms but one sodium atom on the left side and two on the right side.
To get a balanced equation an extra sodium atom is needed on the left, which also gives us an extra oxygen and hydrogen atom. That means there are now two water molecules on the right side and the equation is balanced as H2SO4 + 2NaOH = Na2SO4 + 2H2O. The equation adheres to the Law of Conservation of Mass.
Using the Gram-per-Mole Conversion Factor
A balanced equation is useful to show how many atoms are needed in a chemical reaction, but it doesn't say how much of each substance is required or how much is produced. The balanced equation can be used to express the quantity of each substance in moles, moles of any substance having the same number of atoms.
For example, when sodium reacts with water, the reaction produces sodium hydroxide and hydrogen gas. The unbalanced chemical equation is Na + H2O = NaOH + H2. The right side of the equation has a total of three hydrogen atoms because the hydrogen gas molecule is made up of two hydrogen atoms. The balanced equation is 2Na + 2H2O = 2NaOH + H2.
This means two moles of sodium with two moles of water will produce two moles of sodium hydroxide and a mole of hydrogen gas. Most periodic tables will give the grams per mole for each element. For the reaction above these are sodium: 23, hydrogen: 1 and oxygen: 16. The equation in grams states that 46 grams of sodium and 36 grams of water will react to form 80 grams of sodium hydroxide and 2 grams of hydrogen. The number of atoms and the weights are the same on both sides of the equation, and the grams-per-mole conversion factors can be found in all stoichiometric calculations involving weight.