When a \(0.200 \mathrm{g}\) sample of \(\mathrm{Mg}\) is heated in air, \(0.315 \mathrm{g}\) of product is obtained. Assume that all the Mg appears in the product. (a) If the product were pure \(\mathrm{MgO}\), what mass should have been obtained? (b) Show that the 0.315 g product could be a mixture of \(\mathrm{Mg} \mathrm{O}\) and \(\mathrm{Mg}_{3} \mathrm{N}_{2}.\) (c) What is the mass percent of \(\mathrm{MgO}\) in the \(\mathrm{MgO}-\mathrm{Mg}_{3} \mathrm{N}_{2}\) mixed product?

Understanding the molar mass of a substance is crucial for solving stoichiometry problems in chemistry. Molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol), and it is the sum of the atomic masses of all the atoms in a molecule.

For instance, magnesium (Mg) has an atomic mass of approximately 24.305 g/mol. When you're given the mass of a substance, you divide it by the molar mass to find the number of moles. In our exercise, with a sample of magnesium weighing 0.200 g, the number of moles of Mg is calculated by dividing this mass by the molar mass of Mg, resulting in approximately 0.00823 moles.

Knowing how to calculate the molar mass and moles of a substance is foundational for subsequent calculations in stoichiometry, such as determining theoretical yields and mass percent compositions.

Theoretical yield is a key concept in chemistry that refers to the maximum amount of product that can be generated from a given amount of reactants under perfect conditions. It is calculated by assuming that the reaction goes to completion and that there are no losses or side reactions.

In the exercise, to find the theoretical yield of MgO, we used the molar mass of MgO (40.304 g/mol) and the moles of Mg available to calculate that if all the Mg turned into MgO, 0.332 g of MgO would be formed. This theoretical yield is vital to compare with the actual yield, which is the amount of product actually obtained from the reaction. Any discrepancy between the two can help identify if other compounds, such as Mg₃N₂, might have also formed during the reaction.

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds. In a chemical reaction, the reactants interact to produce new substances, called products. For example, when magnesium is heated in air, it can react with oxygen to form magnesium oxide (MgO), or with nitrogen to form magnesium nitride (Mg₃N₂).

Understanding the reactants and the possible products is essential in stoichiometry. This knowledge allows us to perform calculations, like in our exercise, where we need to consider that a mixture of products might form. Recognizing that not all of the Mg might be converted into MgO is an example of how chemical knowledge informs our stoichiometric calculations.

Mass percent composition is a concept that reflects the concentration of a particular substance within a mixture. It is expressed as the mass of the component of interest divided by the total mass of the mixture, multiplied by 100 to get a percentage.

In our example, the use of the mass percent composition allowed us to determine the percentage of MgO in the final product. Given the actual mass of MgO as 0.315 g and the theoretical mass as 0.332 g, the percent composition calculation reveals that the product is approximately 94.88% MgO by mass. This figure can indicate the purity of the product or can help in identifying how much of another substance, like Mg₃N₂, might be present in a mixture.