Suppose \(12.0 \mathrm{~g}\) of silicon (Si) reacts with \(70.0 \mathrm{~g}\) of oxygen \(\left(\mathrm{O}_{2}\right)\) to give \(25.6 \mathrm{~g}\) of the compound silicon dioxide \(\left(\mathrm{SiO}_{2}\right) .\) In the process, all the silicon gets used up, but some oxygen is left over. (a) For the law of conservation of matter to be obeyed, how much oxygen is unused? (b) What is the percent \(\mathrm{Si}\) in \(\mathrm{SiO}_{2} ?\) (c) What is the percent \(\mathrm{O}\) in \(\mathrm{SiO}_{2}\) ?

Understanding the Law of Conservation of Mass is fundamental to conducting chemical reaction calculations. This principle, articulated by Antoine Lavoisier in the 18th century, states that mass is neither created nor destroyed in a chemical reaction. In practical terms, this means the mass of the reactants must equal the mass of the products in a closed system.

In our exercise, when silicon (Si) reacts with oxygen (O₂), the mass of the reactants (Si and the used portion of O₂) should equal the mass of the product, silicon dioxide (SiO₂). Any discrepancy would suggest excess reactants were present, as confirmed by the 56.4 grams of unused oxygen calculated in the provided solution. This excess does not violate the law since it simply did not participate in the reaction.

The percent composition of a compound provides insight into the proportion of each element within the compound, based on mass. To find this, you must first understand the concept of molar mass, which represents the mass of one mole of a substance.

In the case of silicon dioxide (SiO₂), once we know its molar mass, we determine the percent composition of each element by dividing the element's contribution to the molar mass by the total molar mass of the compound, and then multiplying the result by 100 to get a percentage. For example, in our solution, silicon makes up 46.67% of SiO₂ while oxygen makes up 53.33%. Reflecting on these percentages ensures a clear grasp of the composition of molecules, which is critical in many areas of chemistry, including stoichiometry, chemical analysis, and material science.

The molar mass is another cornerstone concept in chemistry, primarily used to bridge the gap between the atomic scale and the practical world of measuring masses in the lab. It is defined as the mass of one mole of a substance (usually in grams per mole, g/mol) and can be calculated by adding together the atomic masses of the elements in the molecule (from the periodic table), multiplied by their respective number of atoms in the formula.

For instance, in determining the molar mass of silicon dioxide (SiO₂), our solution combined the molar mass of one mole of silicon (28 g/mol) and the molar mass of two moles of oxygen (2 * 16 g/mol), resulting in a molar mass of 60 g/mol for SiO₂. Grasping molar mass calculations is essential for quantifying substances involved in chemical reactions and supports the accurate interpretation of reaction stoichiometry.