If we know the empirical formula of a compound, what additional information do we need to determine its molecular formula?

The empirical formula represents the simplest ratio of the elements in a compound, with the atoms of each element reduced to the lowest whole numbers. For instance, water (H₂O) has an empirical formula of HO because it has a 2:1 ratio of hydrogen to oxygen atoms.

When computing an empirical formula, it’s essential to convert the masses of each element into moles using their atomic weights. Thereafter, one must find the simplest mole ratio of the elements and turn that into whole numbers if they're not already — this process often involves dividing by the smallest number of moles for all the elements present in the compound. This gives the proportions of elements in a molecule without indicating the exact number of atoms.

The molecular formula is a next-level detail compared to the empirical formula. It gives the actual numbers of atoms of each element in a molecule of the compound. Unlike the empirical formula’s simplified ratio, the molecular formula reflects the exact composition.

To illustrate, while the empirical formula for benzene is CH, indicating a carbon to hydrogen ratio of 1:1, the molecular formula is C₆H₆, specifying that each molecule of benzene consists of 6 carbon atoms and 6 hydrogen atoms. Understanding molecular formulas is important for recognizing the identity of substances and for predicting the chemical properties and the behavior of molecules during reactions.

Molar mass is a critical concept in chemistry that bridges together the empirical and molecular formulas. It is the mass of one mole of a substance and is usually expressed in grams per mole (g/mol).

To calculate molar mass, one must sum the atomic weights of all atoms in a compound based on its molecular or empirical formula. For example, the molar mass of water (H₂O) is approximately 18.015 g/mol, calculated by adding the atomic weights of two hydrogen atoms (about 1.008 g/mol each) and one oxygen atom (about 15.999 g/mol). The molar mass is imperative in determining how many molecules a given mass of compound contains and thus links the quantities measured in the lab to the chemical formula of a substance.

Stoichiometry is the mathematical relationship between the relative quantities of reactants and products in chemical reactions. It's based on the conservation of mass and the concept of moles.

In stoichiometry, chemical equations are balanced to reflect the same number of each type of atom on both sides of the reaction. It allows the calculation of the quantities (mass, volume, number of moles) needed or produced in a reaction. For example, if a reaction states that 2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water (2H₂ + O₂ -> 2H₂O), stoichiometry provides the tools to find out how many grams of water are produced from a certain mass of hydrogen. It is fundamental for predicting reaction yields and for scaling up reactions from laboratory to industrial scale.