Determine the number of moles (of molecules or formula units) in each sample. a. 55.98 g CF2Cl2 b. 23.6 kg Fe(NO3)2 c. 0.1187 g C8H18 d. 195 kg CaO

Understanding the concept of molar mass is crucial for various calculations in chemistry. Essentially, molar mass provides a bridge between the mass of a substance and the number of particles it contains - specifically, moles. The molar mass of a compound is determined by adding together the atomic masses of all the atoms in the compound according to its chemical formula. This is usually expressed in grams per mole (g/mol).

For instance, the molar mass of CF2Cl2 is calculated by taking 1 carbon (C) at 12.01 g/mol, 2 fluorines (F) at 19.00 g/mol each, and 2 chlorines (Cl) at 35.45 g/mol each. These individual atomic masses are available on the periodic table and by summing these up, we obtain the total molar mass of the compound. Similarly, for Fe(NO3)2, calculating the molar mass involves summing the atomic mass of iron (Fe), nitrogen (N), and oxygen (O) according to the stoichiometry of the compound. It's a pivotal first step in mole calculations.

Stoichiometry is an area of chemistry that involves the quantitative relationships between the reactants and products in chemical reactions. It's based on the conservation of mass and the concept of the mole, allowing chemists to predict the amounts of substances consumed and produced in a reaction.

When we use stoichiometry to solve problems, like in the given exercise, we often start by understanding the balanced chemical equation and then proceed to use conversion factors that are derived from the equation. For instance, calculating the number of moles of a substance provided in mass requires an understanding of the stoichiometry of the substance's formula, which was illustrated in calculating the molar mass of compounds like CF2Cl2 and Fe(NO3)2 as a preliminary step.

The conversion of mass to moles is a fundamental skill in chemistry, underpinning the concept of stoichiometry. To convert the mass of a substance to the number of moles, we use the substance’s molar mass as a conversion factor.

The formula used is: \[ \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \] When you have the mass of a compound, like C8H18, and the molar mass calculated, you can directly apply this formula. For substances provided in kilograms, such as in the case of CaO, the mass must first be converted to grams since the molar mass is typically expressed in g/mol. This critical step ensures accurate calculation of moles.

An atomic mass unit (amu) is a unit used to express the mass of atoms and molecules. It's defined as one-twelfth the mass of a carbon-12 atom. Atomic mass units provide a convenient way to represent the mass of an atom on a scale that's appropriate for atoms, as they are extremely light.

On the periodic table, the atomic mass of each element is usually stated in amu; for instance, hydrogen has an atomic mass of approximately 1 amu. When it comes to larger chemical compounds, like C8H18, the atomic mass units are summed and then typically converted into grams per mole for practical chemistry applications. This conversion is fundamental for translating the microscopic scale of atoms to the macroscopic scale necessary for laboratory and real-world applications.