Determine the chemical formula of each compound and use it to calculate the mass percent composition of each constituent element. a. phosphorus pentachloride b. nitrogen triiodide c. carbon dioxide

Understanding chemical formulas is fundamental to grasping the composition of compounds. A chemical formula, such as PCl₅ for phosphorus pentachloride, presents the elements involved and the number of atoms of each element in a single molecule of the compound. The subscript numbers, like the '5' in PCl₅, denote the count of atoms, whereas a lack of subscript indicates a single atom is present, as seen with the 'P' for phosphorus. Consequently, nitrogen triiodide is notated as NI₃, representing one nitrogen atom bound to three iodine atoms. In carbon dioxide or CO₂, the formula conveys that each molecule consists of one carbon atom and two oxygen atoms.

To calculate molar mass, sum the atomic masses of all atoms comprising a molecule, as found on the periodic table. Take phosphorus pentachloride, for instance; its molar mass is the sum of the atomic mass of one phosphorus atom plus five chlorine atoms. The resulting molar mass is a crucial stepping stone for calculating mass percentages, as it gives the weight of one mole of the substance. To effectively perform these calculations, it is vital to understand the distinct atomic weights of each atom and accurately multiply these by the number of atoms present in the molecule.

Atomic mass, expressed in atomic mass units (amu) or grams per mole (g/mol), is the weight of a single atom of an element. Typically sourced from the periodic table, these values, such as 14.01 g/mol for nitrogen or 16.00 g/mol for oxygen, are based on the weighted average of all the isotopes of that element. Accurate understanding and usage of atomic masses are integral to the various calculations in chemistry, including finding the molar mass and determining mass percent composition of compounds.

Mass percent composition details the proportion, by mass, of each element within a compound. It's determined by dividing the total mass of each element in a single mole of the compound by the molar mass of the compound, then multiplying by 100. In this exercise, for each compound, we calculated the mass of the constituent atoms and divided by the compound's molar mass. Nitrogen's mass percent in NI₃, for example, is computed by dividing the atomic mass of nitrogen by the molar mass of NI₃ and then multiplying by 100 to yield nitrogen's percentage of the total mass.

Stoichiometry links the quantitative relationships between reactants and products in chemical reactions. It utilizes the principles discussed here—like the chemical formulas and molar masses—to predict the amounts of substances consumed and created in reactions. Understanding the stoichiometric coefficients (the numbers in front of molecules in a balanced equation) and how they relate to moles is key. This allows chemists to calculate how much reactant is needed to produce a desired amount of product or how much product can be made from given reactants. While not directly applied in the mass percent composition calculation, stoichiometry is a broader concept that encompasses these calculations and extends to balance and interpret chemical equations.