How many moles of compound are in each of the following samples: $$ \begin{array}{l}{\text { a. } 6.60 \mathrm{g}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \text { ( molar mass of }} \\\ {\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}=132.17 \mathrm{g} / \mathrm{mol} )} \\ {\text { b. } 4.5 \mathrm{kg} \text { of } \mathrm{Ca}(\mathrm{OH})_{2} \text { (molar mass of }} \\\ {\mathrm{Ca}(\mathrm{OH})_{2}=74.10 \mathrm{g} / \mathrm{mol} )}\end{array} $$

Stoichiometry is akin to a recipe for chemistry. It's the calculation of reactants and products in chemical reactions. It's a core aspect of chemical equations and helps to predict the outcomes of reactions. In essence, it allows us to quantify the exact amounts of substances that will react and the products that will be generated.

As seen in the original exercise, stoichiometry is applied when we calculate how many moles of a compound we have, given its mass and molar mass. This calculation forms the basis for reacting amounts of different substances. For instance, if you know how many moles of one reactant you have, stoichiometry can help you calculate how much of another reactant you'll need to fully react with it, or how much product will be formed.

The mole concept is a bridge between the atomic world and the laboratory world. One mole of any substance contains the same number of entities (Atoms, molecules, ions, etc.) as there are atoms in 12 grams of carbon-12, which is approximately 6.022 x 10^23 entities, known as Avogadro's number.

The mass of one mole of a substance is its molar mass, which is the mass in grams of one mole of the substance. The molar mass is numerically equal to the substance's average atomic or molecular weight in unified atomic mass units (u). In the given sample solutions, we're using the mole concept to convert between mass and number of moles using the compound's molar mass as the conversion factor.

Importance of Chemical Formulas

Chemical formulas like \(\mathrm{(NH_4)_2SO_4}\) and \(\mathrm{Ca(OH)_2}\) represent the composition of chemical compounds. They tell us the elements present and in what ratio they are combined. Understanding chemical formulas is fundamental for stoichiometry as it allows us to identify the number of moles of each element in a given mass of compound.

Reading Chemical Formulas

For example, the chemical formula \(\mathrm{(NH_4)_2SO_4}\) indicates that one molecule of ammonium sulfate contains 2 ammonium ions \(\mathrm{NH_4}^+\) and 1 sulfate ion \(\mathrm{SO_4}^{2-}\). It helps us understand the stoichiometry of a reaction involving ammonium sulfate because it provides the ratio of nitrogen, hydrogen, sulfur, and oxygen atoms present in the compound.

Unit conversion in chemistry is vital for interpreting and manipulating scientific data. It allows us to convert between units like kilograms to grams, liters to milliliters, or atoms to moles. Accurate unit conversion is crucial in stoichiometry to match the units used in formulas.

In the exercise solution, the conversion from kilograms to grams was necessary to use the molar mass in grams per mole to find the number of moles. Remembering common conversion factors, such as 1 kg equals 1000 g, simplifies the process of converting between units and ensures that calculations are done correctly.